A body of an oil jet including: an oil supply port which opens into an oil passage in a cylinder block of an internal combustion engine; a cylinder one end of which is communicated with the oil supply port and the other end of which is closed; and an oil injection port which opens on a side surface of the cylinder. A piston valve is accommodated in the cylinder. The piston valve forms in the cylinder a differential pressure room which is a closed compartment. Moreover, an orifice which makes the differential pressure room being communicated with a side of the oil supply port is formed in the piston valve. The piston valve is biased toward a position at which the oil injection port is closed by a spring. Furthermore, a leak hole which allows oil to be leaked outside of the body from the differential pressure room is formed in the body.