A piston cooling system comprising: a nozzle pipe portion for communicating with an oil passage which is provided in an internal combustion engine and which, in use, is arranged to extend towards an interior of a cylinder bore; and a flow path forming member (30; 40; 50; 60; 90) which is fixed to a distal end portion (22; 322) of the nozzle pipe portion and in which a plurality of oil jetting paths are formed to, in use, thereby cool a piston within the cylinder bore by jetting oil from the oil jetting paths towards a back side of the piston, wherein: the distal end portion comprises an expanded pipe portion (21a; 321a) where the nozzle pipe portion is expanded and the flow path forming member is fittingly inserted into the expanded pipe portion; the flow path forming member has a distal end face (31; 41) which is exposed to an exterior portion at a distal end side of the expanded pipe portion; and the flow path forming member is locked in the expanded pipe portion by deforming a distal end edge (21ae; 321ae) of the expanded pipe portion so as to provide a locking portion (21ak; 321ak) which locks the distal end face of the flow path forming member; characterised in that: the expanded pipe portion has a tapered inner circumferential wall surface which is formed so as to gradually expand towards the distal end edge; the flow path forming member has a tapered outer circumferential wall surface (32w; 42w; 52w; 62w; 92w) which corresponds to the inner circumferential wall surface and groove portions (33ag; 33bg; 33cg) are formed in the outer circumferential wall surface; and the oil jetting paths (33a, 33b, 33c) are formed by the inner circumferential wall surface and the groove portions. a nozzle pipe portion for communicating with an oil passage which is provided in an internal combustion engine and which, in use, is arranged to extend towards an interior of a cylinder bore; and a flow path forming member (30; 40; 50; 60; 90) which is fixed to a distal end portion (22; 322) of the nozzle pipe portion and in which a plurality of oil jetting paths are formed to, in use, thereby cool a piston within the cylinder bore by jetting oil from the oil jetting paths towards a back side of the piston, wherein: the distal end portion comprises an expanded pipe portion (21a; 321a) where the nozzle pipe portion is expanded and the flow path forming member is fittingly inserted into the expanded pipe portion; the flow path forming member has a distal end face (31; 41) which is exposed to an exterior portion at a distal end side of the expanded pipe portion; and the flow path forming member is locked in the expanded pipe portion by deforming a distal end edge (21ae; 321ae) of the expanded pipe portion so as to provide a locking portion (21ak; 321ak) which locks the distal end face of the flow path forming member; the distal end portion comprises an expanded pipe portion (21a; 321a) where the nozzle pipe portion is expanded and the flow path forming member is fittingly inserted into the expanded pipe portion; the flow path forming member has a distal end face (31; 41) which is exposed to an exterior portion at a distal end side of the expanded pipe portion; and the flow path forming member is locked in the expanded pipe portion by deforming a distal end edge (21ae; 321ae) of the expanded pipe portion so as to provide a locking portion (21ak; 321ak) which locks the distal end face of the flow path forming member; characterised in that: the expanded pipe portion has a tapered inner circumferential wall surface which is formed so as to gradually expand towards the distal end edge; the flow path forming member has a tapered outer circumferential wall surface (32w; 42w; 52w; 62w; 92w) which corresponds to the inner circumferential wall surface and groove portions (33ag; 33bg; 33cg) are formed in the outer circumferential wall surface; and the oil jetting paths (33a, 33b, 33c) are formed by the inner circumferential wall surface and the groove portions. the expanded pipe portion has a tapered inner circumferential wall surface which is formed so as to gradually expand towards the distal end edge; the flow path forming member has a tapered outer circumferential wall surface (32w; 42w; 52w; 62w; 92w) which corresponds to the inner circumferential wall surface and groove portions (33ag; 33bg; 33cg) are formed in the outer circumferential wall surface; and the oil jetting paths (33a, 33b, 33c) are formed by the inner circumferential wall surface and the groove portions.