Disclosed is a small rotary cylinder device wherein friction loss is reduced and energy is conserved by reducing effects of a reaction force applied from cylinders to pistons, which is incorporated in a piston composite body and which linearly reciprocates, the piston composite body being eccentrically connected to a first crank shaft rotating about a shaft. Guide bearings (1c) guide linear reciprocation of first and second piston assemblies (7, 8) attached to second tube bodies (6b), which are caused by rotating a first crank shaft (5) about a shaft (4) and rotating a piston composite body (P) about the first crank shaft (5), in the radial direction of a rolling circle of second imaginary crank shafts (14a, 14b), which has a radius 2r and which is centered at the shaft (4).