To provide a back support device which can be easily mounted on a snow board equipped with an existing step in type boot binding device.

This back support device comprises a plate 1 for mounting fixed between a boot binding device B and a snow board S and a back support member 2 which is detachably fixed on the plate 1 for mounting and comes in contact with an area above the heel of each boot Y for the snow board from behind it. The plate 1 for mounting is provided with screw holes corresponding to mounting screws of a base plate 51 of the boot binding device B while having a pair of right and left rising walls 5 and 5 set to a height allowing the right-wise and left-wise inclination of the boot Y facing right and left sides of the boot Y. The back support member 2 is screwed down on the rising walls 5 and 5.