Provided is a cylinder device provided with a mechanism for detecting, in three stages, the position of an output member by using the outer periphery of a piston rod of the output member, and the inner wall of a housing. A housing (2) is provided with an A passage (Ap) for supplying compressed gas, and a B passage (Bp) for discharge to the atmosphere. A piston rod (3) is provided with a projection (12a) that pulls and pushes down the seal member (11) together with an annular valve member (10). The hollow inner wall inside the housing is provided with a sliding region (13) having an inner diameter such that a seal member (11a) makes contact therewith to effectively enable the seal function, a loose insertion region (14) having a large inner diameter such that the sealing function of the seal member (11a) does not work at all and a valve member region (18) which has a large inner diameter such that the annular valve member (10) can move up/down, and which stores the annular valve member (10) while the upper side is surrounded by a ring surface (2d), the valve member region communicating with the B passage (Bp).