To provide a mat that can prevent a buttock slip forward by its bulky member which enables the user of a wheelchair maintain a stable and comfortable posture.

The mat is composed of a flat cushion member 5 that is placed on a seat 2 of a chair, a gelatinous member 9 that is laid under a buttock seating part 8 of the cushion member, and a bulky member 12 that is detachably installed on the surface of a thigh seating part 11 of the cushion and makes the height of the thigh seating part higher than the buttock seating part 8. The bulky member is composed of a large number of bulky member parts 13 that are divided at parting lines 16 that run to the longitudinal direction from a sitter and by arbitrarily changing the number of the bulky member parts across directional width D of the bulky member is changeable.