A rotary piston engine , comprising: a rotor housing with a trochoid inner circumferential surface ; side housings , 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 , 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 . <EOS>