In a side-valve internal combustion engine, a combustion chamber in a cylinder head is defined into a substantially triangular pyramid shape having a triangular opened edge. A third angle portion of the opened edge formed by a first slant having a relatively small angle and a second slant having a relatively large angle is offset from an axis of a cylinder bore toward the first slant. Thus, it is possible to a swirl of an air-fuel mixture during an intake stroke to uniformize the air-fuel ratio, thereby providing a reduction in fuel consumption, and producing a swirl of combustion gas during an expansion stroke to eliminate the non-uniformity of combustion, thereby providing an increase in power output and a reduction in amount of unburned components in the exhaust gas.