A rotary type cylinder device, which is capable of dealing with interconversion of reciprocating motions of pistons (7, 8) in cylinders and a rotary motion of a shaft, comprising: a rotatable shaft; a crank shaft which is eccentrically provided with respect to an axis of the rotatable shaft, whereby the crank shaft is revolved around the rotatable shaft on an orbit of radius of r from the rotatable shaft; an eccentric cylindrical body which is constituted by a first cylindrical section to which the crank shaft is coaxially and relatively rotatably fitted, and second cylindrical sections, whose axes (14a, 14b) are eccentric with respect to the axis of the first cylindrical section and which extend from both axial ends of the first cylindrical section, the axes of the second cylindrical sections revolving around the axis of the crank shaft on an orbit of radius of r; a first piston unit and a second piston unit which are attached to the respective second cylindrical sections of the eccentric cylindrical body, the piston units (7, 8) crisscrossing with each other; first and second balance weights (9, 10) for producing rotational mass balance around the rotatable shaft, the first and second balance weights (9, 10) being respectively provided to both end parts of the crank shaft; and a main body case holding the rotatable shaft, the main body case accommodating the crank shaft, the first balance weight and the second balance weight, the eccentric cylindrical body, and the first piston unit and the second piston unit, wherein the eccentric cylindrical body, and the first piston unit and the second piston unit define a composite piston assembly, characterized in that rotational mass balances relating to (i) the first and second piston units (7, 8) around the second cylindrical sections, (ii) the composite piston assembly around the crank shaft, and (iii) the crank shaft and the composite piston assembly around the rotatable shaft are uniformly produced by only the first and second balance weights (9, 10) which are attached to the both end parts of the crank shaft first pinholes are formed in both end parts of the crank shaft respectively, the axes of the first pinholes being perpendicular to the axis of the crank shaft, axial holes and second pinholes (9b, 10b) are formed in shaft sections of the first and second balance weights (9, 10) respectively, the axes of the second pinholes (9b, 10b) being perpendicular to the axes of the axial holes, the both end parts of the crank shaft are respectively fitted in the axial holes of the first and second balance weights (9, 10) in a state where the first pinholes and the second pinholes (9b, 10b) communicate with each other, and pins (11a, 11b) are fitted and retained in the first and second pinholes (5b, 9b; 5b, 10b) so as to integrate the crank shaft with the first and second balance weights (9, 10). a rotatable shaft; a crank shaft which is eccentrically provided with respect to an axis of the rotatable shaft, whereby the crank shaft is revolved around the rotatable shaft on an orbit of radius of r from the rotatable shaft; an eccentric cylindrical body which is constituted by a first cylindrical section to which the crank shaft is coaxially and relatively rotatably fitted, and second cylindrical sections, whose axes (14a, 14b) are eccentric with respect to the axis of the first cylindrical section and which extend from both axial ends of the first cylindrical section, the axes of the second cylindrical sections revolving around the axis of the crank shaft on an orbit of radius of r; a first piston unit and a second piston unit which are attached to the respective second cylindrical sections of the eccentric cylindrical body, the piston units (7, 8) crisscrossing with each other; first and second balance weights (9, 10) for producing rotational mass balance around the rotatable shaft, the first and second balance weights (9, 10) being respectively provided to both end parts of the crank shaft; and a main body case holding the rotatable shaft, the main body case accommodating the crank shaft, the first balance weight and the second balance weight, the eccentric cylindrical body, and the first piston unit and the second piston unit,