To provide an overlay mattress for preventing bed sore, which is constituted to be applicable to reference for evaluating measures against the bed sore.

A soft base material 5 (e.g. a urethane foam material) having size corresponding to a basic mattress 4 and lower repulsion compared with a base material which is used as the filling of the basic mattress 4, is covered by a water-proof and extending/contracting outer material 6 to constitute a base flat part 1a being a part near the center of a bed with a prescribed thickness (e.g. 30-70 mm). The overlay mattress 1 for preventing bed sore is provided with the part 1a and side thick parts 1b wherein parts near the side surfaces in a bed width direction are formed to be thicker than the other parts. The water-proof outer material 6 with extending/contracting property is removably arranged on the basic mattress 4 by a fitting means 8. A urethane foam material with apparent density of 10 kg/m3-100 kg/m3, and compression hardness of 10 N-400 N is used as the soft base material 5.