A mounting structure for a piston cooling oil jetting device, including a piston cooling oil jetting device fastened and fixed to a crankcase by a screw engagement member, the piston cooling oil jetting device comprising: a main body portion having an oil channel communicating with an oil passage provided in the crankcase; a nozzle portion extending from the main body portion toward a piston in a cylinder bore, the nozzle portion having an oil jet port; and a fastening arm portion provided to project from the main body portion, the fastening arm portion formed therein with an insertion hole through which the screw engagement member is inserted and passed; characterized in that the crankcase is provided with an arm portion mounting boss to which the screw engagement member is fastened, and a main body portion mounting hole into which the main body portion is mounted, a fastening surface of the arm portion mounting boss and a main body portion mounting surface of the main body mounting hole are formed on planes different in height with respect to an axial direction of the screw engagement member taken as a height direction, and the fastening surface and the main body portion mounting surface are connected to each other through a contact surface portion extending along the axial direction of the screw engagement member; and wherein the fastening arm portion is formed along the contact surface portion of the arm portion mounting boss, at least one part of the fastening arm portion is in contact with the contact surface portion in a state where the piston cooling oil jetting device is fixed to the crankcase, and one end part of the fastening arm portion is fastened to the fastening surface. a main body portion having an oil channel communicating with an oil passage provided in the crankcase; a nozzle portion extending from the main body portion toward a piston in a cylinder bore, the nozzle portion having an oil jet port; and a fastening arm portion provided to project from the main body portion, the fastening arm portion formed therein with an insertion hole through which the screw engagement member is inserted and passed; characterized in that the crankcase is provided with an arm portion mounting boss to which the screw engagement member is fastened, and a main body portion mounting hole into which the main body portion is mounted, a fastening surface of the arm portion mounting boss and a main body portion mounting surface of the main body mounting hole are formed on planes different in height with respect to an axial direction of the screw engagement member taken as a height direction, and the fastening surface and the main body portion mounting surface are connected to each other through a contact surface portion extending along the axial direction of the screw engagement member; and wherein the fastening arm portion is formed along the contact surface portion of the arm portion mounting boss, at least one part of the fastening arm portion is in contact with the contact surface portion in a state where the piston cooling oil jetting device is fixed to the crankcase, and one end part of the fastening arm portion is fastened to the fastening surface.