Abstract: A rotary piston engine has a rotor housing and a rotor disposed therein for planetary rotary motion. The housing and the rotor cooperate to define a plurality of variable volume working chambers each of which is moved along the inner peripheral surfaces of the housing and acts as intake, compression, combustion-expansion and exhaust chambers as the rotary planetary motion proceeds. A plug hole in which electrodes of an ignition plug are disposed is formed in the housing and so positioned as to be communicated with the intake chamber at its final stage of intake stroke. A passage is formed in the housing and connected at one end to the plug hole. The other end of the passage is connected to a supply source of a combustible gas, such as LPG or air-gasoline mixture, or to the atmosphere.