An ascent-detecting air supply port (33), a descent-detecting air supply port (43), and an air discharge port (44) are provided to a housing (2). The ascent-detecting air supply port (33) is communicatively connected to an air outlet (38) by an ascent-detecting air passage (39). The descent-detecting air supply port (43) is communicatively connected to the air discharge port (44) by a descent-detecting air passage (45). The descent-detecting air passage (45) is closed by a valve mechanism (49) including: a valve surface (50) provided on a piston portion (15); and a valve seat (51) provided on an inner surface of the housing (2), when an advance piston (14) is retracted to a lowered position.