The principle of the thermal storage in building mass (TSBM) is storing thermal energy in building thermal mass, such as concrete slab, at night and discharging thermal energy naturally in daytime. It is expected that the thermal storage will be effective for reducing the heating load at the beginning of the operation in a cold region. Adding to that, the indoor thermal environment will improve because it heats up the cold floor slab directly. The simulation model was constituted to recreate features of TSBM, which were observed in a field survey in an office building. It was estimated by simulation that the maximum heating load at the beginning of the operation would be reduced in spite of the several-percent increase of the total heating load and that the temperature around the foot would rise. The balance of thermal environment and thermal load was considered from the simulation result and appropriate TSBM operation was proposed.