A rotary piston engine, comprising: a rotor housing with a trochoid inner circumferential surface; side housings (4, 5), each being disposed on either side of the rotor housing along an axis of the rotor housing, and forming a rotor housing chamber together with the rotor housing; a substantially triangular rotor housed in the rotor housing chamber, having three outer circumferential surfaces that partition the rotor housing chamber into three operation chambers, having recesses each being formed in one of the outer circumferential surfaces, and allowing the operation chambers to sequentially perform intake, compression, expansion and exhaust strokes while circumferentially moving the operation chambers by rotation; and a spark plug provided in the rotor housing, characterised by the fact that each of the recesses includes: a leading-side area extending forward from a longitudinal center on the associated one of the outer circumferential surfaces in a rotation direction of the rotor, and a trailing-side area continuous with the leading-side area, and extending rearward from the longitudinal center in the rotation direction, the leading-side area has a width gently increasing toward a front in the rotation direction of the rotor, and an arc-like distal end, the trailing-side area is continuous with a narrowed proximal end of the leading-side area, extends, with a same width as the proximal end, rearward in the rotation direction of the rotor, and has a tapered distal end and a distal edge that extends linearly along a width of the rotor, forward extension of the leading-side area is longer than rearward extension of the trailing-side area, and the leading-side area has a larger volume than the trailing-side area. a rotor housing with a trochoid inner circumferential surface; side housings (4, 5), each being disposed on either side of the rotor housing along an axis of the rotor housing, and forming a rotor housing chamber together with the rotor housing; a substantially triangular rotor housed in the rotor housing chamber, having three outer circumferential surfaces that partition the rotor housing chamber into three operation chambers, having recesses each being formed in one of the outer circumferential surfaces, and allowing the operation chambers to sequentially perform intake, compression, expansion and exhaust strokes while circumferentially moving the operation chambers by rotation; and a spark plug provided in the rotor housing, characterised by the fact that each of the recesses includes: a leading-side area extending forward from a longitudinal center on the associated one of the outer circumferential surfaces in a rotation direction of the rotor, and a trailing-side area continuous with the leading-side area, and extending rearward from the longitudinal center in the rotation direction, the leading-side area has a width gently increasing toward a front in the rotation direction of the rotor, and an arc-like distal end, the trailing-side area is continuous with a narrowed proximal end of the leading-side area, extends, with a same width as the proximal end, rearward in the rotation direction of the rotor, and has a tapered distal end and a distal edge that extends linearly along a width of the rotor, forward extension of the leading-side area is longer than rearward extension of the trailing-side area, and the leading-side area has a larger volume than the trailing-side area.