[en] As part of the research and development program on geological disposal of high-level radioactive waste (HLW), the Horonobe Underground Research Center, a division of the Japan Atomic Energy Agency (JAEA), is implementing the Horonobe Underground Research Laboratory Project (Horonobe URL Project) with the aim at investigating sedimentary rock formations. According to the research plan described in the Midterm Plan of JAEA, geological investigations are to be carried out during the drilling of a shaft down to intermediate depth, while research and development in the areas of engineering technology and safety assessment are to be promoted by collaboration with other research organizations. The results of the R and D activities will be systematized as a 'knowledge base' that supports a wide range of arguments related to the safety of geological disposal. The Horonobe URL Project is planned to extend over a period of 20 years. The investigations will be conducted in three phases, namely 'Phase 1: Surface-based investigations', 'Phase 2: Construction phase' (investigations during construction of the underground facilities) and 'Phase 3: Operation phase' (research in the underground facilities). This report summarizes the investigation program for the 2008 fiscal year (2008/2009), the 4th year of the Phase 2 investigations. In the 2008 fiscal year, investigations in geoscientific research', including 'development of techniques for investigating the geological environment', 'development of techniques for long-term monitoring of the geological environment', 'development of engineering techniques for use in the deep underground environment' and studies on the long-term stability of the geological environment', are continuously carried out. Investigations in 'research and development on geological disposal technology', including 'improving the reliability of disposal technologies' and 'enhancement of safety assessment methodologies', are also continuously carried out. Construction of the underground facilities is ongoing at Ventilation Shaft and Access Shaft (East). Pre-boring close to Ventilation Shaft is also continued. Regarding the surface facilities, construction of the International Communication House will be started. (author)