A fluid pressure cylinder (2; 2A) comprising a cylinder main body (10; 10A) in which a cylindrical hole is formed, a piston member (4; 4A) having a piston portion (5; 5A) movably received in the cylindrical hole and an output rod (6; 6A) that extends from the piston portion to the exterior of the cylinder main body, and a fluid pressure operation chamber (12a, 12b) formed in the cylindrical hole, comprising: an auxiliary rod (7; 7a) that extends from the piston portion in an opposite direction to the output rod; a rod insertion hole that is formed in the cylinder main body so as to communicate with the fluid pressure operation chamber and into which the auxiliary rod can be inserted; a first open/shut detection valve mechanism (11; 11A), installed in the cylinder main body; and a first fluid passage (32; 32A) formed in the cylinder main body so as to be opened and closed by the first open/shut detection valve mechanism; characterised in that the first open/shut detection valve mechanism comprises: a first valve body reception hole that is formed parallel to an axis of the cylindrical hole in the cylinder main body, and is communicated with the fluid pressure operation chamber; a first valve body that is movably received in the first valve body reception hole and has a first concave engagement portion (37; 37M) on an external circumferential portion thereof; a first spherical body that is movably fitted in the cylinder main body and is capable of engaging to the first concave engagement portion; and a first engagement portion (39, 53; 53A) formed on the auxiliary rod and to which the first spherical body can engage; wherein, when fluid pressure is charged into the fluid pressure operation chamber and the piston member has reached its movement limit position, the first valve body receives a fluid pressure in the fluid pressure operation chamber, and a portion of the first spherical body is engaged to the first engagement portion and the first valve body is changed over to a valve-closed position; and when the piston member has shifted from its movement limit position, the first spherical body does not engage with the first engagement portion and engages with the first concave engagement portion so that the first valve body maintains a valve-open position. an auxiliary rod (7; 7a) that extends from the piston portion in an opposite direction to the output rod; a rod insertion hole that is formed in the cylinder main body so as to communicate with the fluid pressure operation chamber and into which the auxiliary rod can be inserted; a first open/shut detection valve mechanism (11; 11A), installed in the cylinder main body; and a first fluid passage (32; 32A) formed in the cylinder main body so as to be opened and closed by the first open/shut detection valve mechanism; characterised in that the first open/shut detection valve mechanism comprises: a first valve body reception hole that is formed parallel to an axis of the cylindrical hole in the cylinder main body, and is communicated with the fluid pressure operation chamber; a first valve body that is movably received in the first valve body reception hole and has a first concave engagement portion (37; 37M) on an external circumferential portion thereof; a first spherical body that is movably fitted in the cylinder main body and is capable of engaging to the first concave engagement portion; and a first engagement portion (39, 53; 53A) formed on the auxiliary rod and to which the first spherical body can engage; a first valve body reception hole that is formed parallel to an axis of the cylindrical hole in the cylinder main body, and is communicated with the fluid pressure operation chamber; a first valve body that is movably received in the first valve body reception hole and has a first concave engagement portion (37; 37M) on an external circumferential portion thereof; a first spherical body that is movably fitted in the cylinder main body and is capable of engaging to the first concave engagement portion; and a first engagement portion (39, 53; 53A) formed on the auxiliary rod and to which the first spherical body can engage; wherein, when fluid pressure is charged into the fluid pressure operation chamber and the piston member has reached its movement limit position, the first valve body receives a fluid pressure in the fluid pressure operation chamber, and a portion of the first spherical body is engaged to the first engagement portion and the first valve body is changed over to a valve-closed position; and when the piston member has shifted from its movement limit position, the first spherical body does not engage with the first engagement portion and engages with the first concave engagement portion so that the first valve body maintains a valve-open position.