To perform an optimal change in the operation system (homogeneous combustion operation, stratification combustion operation, homogeneous lean- burn operation).

Operation methods of internal combustion engine and the device are provided. At the time of change from a side of the operation method to the other operation method, in a transition process, an injection fuel mass flow rate and a setting ignition angle are calculated based on a previous operation method and a new operation method. From homogeneous combustion operation to homogeneous lean-burn operation, and/or, at the time of change to homogeneous combustion operation from homogeneous lean-burn operation, in a shift process, the injection fuel mass flow rate to a cylinder injected after the start of a shift process is decided in consideration of a new operation method, establishment of an ignition angle over the cylinder which is not ignited yet is performed in consideration of a previous operation method.