To provide a piston push-in tool provided with a device for detecting abnormality of oil ring assembly.

This piston push-in tool 1 contracts the diameter of the oil ring 111 by pushing a piston 101 with the oil ring 111 previously assembled to an oil ring groove 102 in a cylindrical guide tool 55 provided with a piston insertion hole 57 having a taper surface 58a whose inside diameter is reduced, and is designed that the piston 101 is pushed in the piston insertion hole 57 of the guide tool 55 by a pad 46 through a push-in tool main body 2 and a load cell 43 which are moved by a pushing/pressing applying means P so as to contract the diameter of the oil ring 111, push-in load is detected by the load cell 43, and the detected load is determined whether or not it is within a normal push-in load range by a determination means 61.