To provide a compression ignition engine for realizing premix compression ignition without reducing output even under an operation condition higher in a load than a conventional load.

This compression ignition engine (a diesel engine 1) performs ignition combustion in a combustion chamber 5 after promoting premix of fuel by injecting fuel in the timing earlier than the compression top dead center. The compression ignition engine has an auxiliary chamber 11 arranged in a cylinder head 2 so as to communicate with the combustion chamber 5, a control valve 12 for opening and closing a part between the auxiliary chamber 11 and the combustion chamber 5, a heat removing means (a porous material 17 and a heat radiating plate 18) for externally releasing heat of residual gas in the auxiliary chamber 11, and a control device for opening the control valve 12 in the middle of a compression stroke and closing the control valve just before injecting the fuel.