To provide an automatic reciprocating system continuing reciprocation automatically in a certain cycle by a hydraulic system.

An automatic reciprocating system is provided with a piston 4 contained free to slide in a body 3, a spool 5 which has a contraction position driving the piston 4 in an contraction direction and an expansion position driving the piston 4 in an expansion direction by switching the pressure being guided into the piston 4, and a spool drive pressure chamber 13, 14 contracting during the piston 4 moves. According to pressure of the spool pressure chamber 13, 14, the position of the spool 5 is switched.