In a rotary piston internal combustion engine comprising a center housing having a trochoidal inner peripheral surface, side housings provided at both sides of said center housing, a three apexed rotary piston is provided through an eccentric shaft within a casing formed by said center and side housings. An oil seal device is disposed in an annular groove formed on a thrust surface of the rotary piston including an oil seal ring having an annular recess or an annular step on the rear portion thereof. A U-shaped elastic seal ring is disposed in said annular recess or step and has two lips slidably and sealingly contacting at least one radially facing surface of said annular recess, said annular step or said annular groove, and a spring is disposed between said oil seal ring and said U-shaped elastic seal ring for pressing said oil seal ring and said U-shaped elastic sealing in opposite axial directions.