An open/shut valve mechanism (11) for detecting that a piston member (4) of a hydraulic pressure cylinder (2) has reached an unclamp position comprises: a valve body reception hole (35) that communicates with a unclamp hydraulic pressure operating chamber (12a), a valve body (36) movably received in the valve body reception hole (35) and having an annular concave engagement portion (37) on its external circumferential portion; a spherical body (38) that is movably installed in a cylinder main body (10) and that can engage with the concave engagement portion (37); and an engagement portion (39) that is formed on an auxiliary rod (7) and with which the spherical body can engage; and when hydraulic pressure is charged into the unclamp hydraulic pressure operating chamber (12a) and the piston member (4) has reached its unclamp position, the valve body (36) receives the hydraulic pressure, a portion of the spherical body (38) engages to the engagement portion (39) and the valve body (36) changes over to the valve closed position; and, when the piston member (4) has shifted from its unclamp position, the spherical body (38) engages with the annular concave engagement portion (37) without engaging to the engagement portion (39), so that the valve body (36) is held in its open position.