Disclosed is a piston cooling device wherein the cooling efficiency of a piston is improved by oil injected from an oil jet and supplied to a cooling passage provided in the piston, and the amount of cooling oil is reduced when an internal combustion engine is operated at maximum output. The piston cooling device is provided with a piston for an internal combustion engine, in which a circumferential passage and a cooling passage having an inlet passage and an outlet passage are provided, and an oil jet for injecting oil from an injection port to the inlet passage. The oil jet injects oil at every stroke of the piston so that a two-phase plug flow composed of gas and oil is formed in the cooling passage at least when an internal combustion engine is operated at maximum output.
