A variable compression device of the present disclosure includes: a piston rod (6); a fluid chamber forming member (7a) which forms a portion of a fluid chamber (R3) configured to move the piston rod in a direction in which a compression ratio is increased by supplying a boosted working fluid thereto; a regulation member (7c) which is configured to regulate movement of the piston rod in a direction in which a compression ratio is increased; and a sliding ring (6d) which is provided on a circumferential surface of the piston rod to be slidable with respect to the fluid chamber forming member, in which a lower space (R7) surrounded by the fluid chamber forming member and the piston rod is formed below the sliding ring, an upper space (R6) surrounded by the fluid chamber forming member, the piston rod, and the regulation member is formed above the sliding ring, and the fluid chamber forming member includes a communication channel (R11) that is configured to guide a portion of the working fluid, which is supplied to the lower space as a coolant, to the upper space.