Disclosed is a flush toilet unit, which is capable of measuring a urine volume with a high degree of accuracy without making a significant change to a conventional water-closet structure. The flush toilet unit (1) of the present invention comprises a bowl (12) for receiving urine of a user therein, a trap portion (15) fluidically connected to the bowl and adapted to guide a pooled water in the bowl to a sewer pipe and to form a water seal relative to the sewer pipe, pooled-water discharge means (34, 35) for lowering a pooled-water level in the bowl to a given water level which is below an overflow water level in the trap portion, water-level measurement means (46) for measuring a water-level variation between the lowered pooled-water level set by the pooled-water discharge means and a water level in the bowl after urination by the user, urine-volume calculation means for calculating a volume of urine voided into the bowl by the user, in accordance with the water-level variation value measured by the water-level measurement means, and water supply means for supplying water into the bowl to return the pooled-water level in the bowl to the overflow water level.