In a diesel engine control device, a normal injection mode that implements normal combustion and a premixed injection mode that implements premixed combustion are set, the normal injection mode is implemented when the actual engine operating conditions are in the normal region, and the premixed injection mode is implemented when the actual engine operating conditions are in the premixed region. When one control mode is being implemented in the corresponding engine operating region, the control mode is forcibly switched to the other control mode when the engine enters a predetermined transition state. This makes it possible to switch earlier than normal, and in particular when accelerating, the desired acceleration performance can be obtained.