A piston structure of an engine includes a piston having a crown part including a crown surface defining a combustion chamber, and an opposite back surface, an oil jet, a center-side first cavity and an outer-circumference-side second cavity formed inside the crown part and partitioned from each other, each cavity including a ceiling surface on the combustion chamber side and an opposite bottom surface, a communicating hole that is formed in a partitioning wall between the cavities, an introduction hole that is formed in the bottom surface of the first cavity and introduces into the first cavity oil injected toward the back surface from the oil jet, and a discharge hole that is formed in the bottom surface of at least one cavity and discharges the oil. The ceiling surface of the second cavity is located on the combustion chamber side of the ceiling surface of the first cavity.