This study establishes an evaluation framework for document-level text simplification in Japanese by constructing a human-annotated dataset and examining the reliability of LLM-based automatic evaluation. We first developed detailed annotation guidelines covering four criteria—necessity, sufficiency, sentence-level simplicity, and document-level simplicity—and collected human ratings for 1,128 source–target document pairs derived from the Wikipedia part of the Japanese simplification corpus JADOS. Using this dataset, we conducted extensive experiments comparing human judgments with evaluations from large language models, including GPT, Claude, and Gemini. The results show that GPT-4o and Gemini 2.5 Pro achieve high agreement with human annotators even in the 0-shot setting, demonstrating their potential as reliable automatic evaluators for Japanese simplification. However, LLMs exhibited a consistent tendency to underestimate document-level simplicity, particularly for kanji-dense texts or texts with relatively long sentences and a small number of sentences. This work provides the first benchmark for evaluating document-level text simplification in Japanese and offers practical evidence that LLM-based evaluation can support scalable assessment for Japanese document-level simplification.

We investigate whether large language models (LLMs) can generate literal usage examples for Japanese multiword expressions (MWEs), whose literal readings are structurally low-frequency in available corpora.Prior work on MWEs has largely focused on detecting idiomatic usages in context, leaving literal usages underrepresented particularly for Japanese MWEs whose literal readings are rare and structurally diverse.Because literal readings are rarely attested in corpora, we design a lexicon-grounded setup that uses corpus non-literal usages as contrastive cues for controlled prompting. We evaluate the generated sentences using automatic literalness judgments and human literalness judgments, together with manual inspection.Our results show that providing contrastive non-literal information stabilizes literal generation and improves quality compared with prompts that include only literal information or no hints. In addition, we conduct an LLM-based understanding test that compares model predictions of literal and idiomatic plausibility with human judgments.The results indicate that the model aligns more closely with human judgments for idiomatic interpretations than for literal ones, highlighting the relative difficulty of modeling literal readings of MWEs.The study demonstrates that LLMs can complement existing resources by supplying frequency-independent literal examples and offers a controlled framework for examining contextual meaning understanding of Japanese MWEs.

The meanings and relationships of words shift over time. This phenomenon is referred to as semantic shift. Research focused on understanding how semantic shifts occur over multiple time periods is essential for gaining a detailed understanding of semantic shifts. However, detecting change points only between adjacent time periods is insufficient for analyzing detailed semantic shifts, and using BERT-based methods to examine word sense proportions incurs a high computational cost. To address those issues, we propose a simple yet intuitive framework for how semantic shifts occur over multiple time periods by utilizing similarity matrices based on word embeddings. We calculate diachronic word similarity matrices using fast and lightweight word embeddings across arbitrary time periods, making it deeper to analyze continuous semantic shifts. Additionally, by clustering the resulting similarity matrices, we can categorize words that exhibit similar behavior of semantic shift in an unsupervised manner.