To provide a compact boosting type hydraulic cylinder capable of, taking out an operation force increased in an optional position, compact in size, long in life and stable in operation in the pushing-out operation of a piston rod.

An auxiliary cylinder 2 for boosting is installed in series outside a head cover 4 of a hydraulic cylinder 1. A space 19 having a tapered surface 19a is formed inside a piston 10. A clutch shaft 7 connected to a boosting piston rod 9 in the auxiliary cylinder 2 is inserted into the space. A locking steel ball 18 is arranged between a clutch shaft outer peripheral surface and the tapered surface 19a inside of the piston 10.