Control unit for spark ignition-type engine

The unit has an operation mode controlling unit for executing a control of a specific operation mode in a partial load range of an engine. A fuel is supplied to a burnt gas with a lean-air fuel ratio. An air fuel ratio controlling unit is controlling the ratio such that concentration of oxygen in exhaust gas discharged from a following cylinder (2B, 2C) is a value corresponding to a combustion state at a stoichiometric ratio. In special operation mode, the fresh air introducing value is closed and the switching valve is controlled so that the valve burnt gas discharged from the preceding cylinder (2A, 2D) introduced in a side of inter-cylinder gas passage (22) into the following cylinder on the intake stroke side.