A piston structure for a four-stroke internal combustion engine is provided, enabling an increase in the intake efficiency and a favorable ignitability to improve the performance of the internal combustion engine. In a piston structure for a four-valve four-stroke internal combustion engine, a piston crown includes: an intake-side inclined flat surface formed in a single plane by bottom surfaces of a pair of intake valve recesses and an intake-side common flat surface; and an exhaust-side inclined flat surface formed in a single plane by bottom surfaces of a pair of exhaust valve recesses and an exhaust-side common flat surface. The intake-side inclined flat surface and the exhaust-side inclined flat surface are formed into a pent-roof shape with a ridge line at an intersection therebetween. The exhaust-side inclined flat surface extends into the intake valve recesses, and the ridge line is located on a side of the intake valve recesses.