A controller (70) controls combustion in an internal combustion engine that injects a fuel having high self-ignitability into a cylinder during a compression stroke, in which a premixed gas formed from a fuel having low self-ignitability is compressed within the cylinder, and ignites the premixed gas using the injected fuel as an ignition source. The controller (70) is programmed to detect any one of a self-ignitability of the fuel forming the premixed gas, a self-ignitability of the fuel serving as the ignition source, and an internal cylinder pressure, and set the other two in accordance with a resulting detection value. Thus, combustion conditions can be controlled appropriately, regardless of the engine operating conditions.