To provide a mat for a bed easy in handling, simple in structure, low in cost, safe and furthermore capable of effectively using the living space, and a sofa bed using the mat.

The mat is formed of two longitudinally extending split mat pieces 2a and 2b connected to each other by pairs of front and back side strip connectors 3a and 3b such that one 2a of the mat pieces can be freely raised up or laid down. When the bed mat is put on a bed body 1 and the mat piece 2a is erected substantially upright, the mat piece 2a is supported by the pairs of upper and lower side strip connectors 3a and 3b to serve as a backrest surface at the rear surface 22a thereof. The upper surface portion of the bed body 1 exposed by erecting the mat piece 2a substantially upright serves as the seat surface.