Provided are a volume change compensation device capable of reducing a manufacturing burden with a simple configuration and a damper device including the volume change compensation device. A damper device 100 includes a rotary damper, and includes a volume change compensation device 140 in a shaft 121 of a rotor 120. The volume change compensation device 140 includes an inner cylinder piston 142 pressed by an inner cylinder piston pressing elastic body 145 in a body tube 141 communicating with a hydraulic fluid housing portion 103 of the damper device 100 through a connection path 141a. The inner cylinder piston 142 is formed in a bottomed cylindrical shape opening on a connection path 141a side. In the inner cylinder piston 142, an inner cylinder inner small piston 143 is pressed against a bottom portion 142b by a small piston pressing elastic body 144. An air hole 142c is formed at the bottom portion 142b of the inner cylinder piston 142. The inner cylinder inner small piston 143 slides in the inner cylinder piston 142 according to the amount of hydraulic fluid 150 in the inner cylinder piston 142.