During lean combustion, an ECU as a control device for a diesel engine calculates a ratio between a target A/F value and a detected A/F value detected by a A/F sensor, wherein the target A/F value is a ratio between a target air volume and a target injection amount in order to obtain a request torque. The ECU calculates the air volume after compensation during rich combustion by multiplying the target air volume with the compensation gain value. The ECU instructs an air intake throttle valve and an EGR valve to regulate those opening degrees according to the calculation result in order to decrease a torque difference between the lean combustion and the rich combustion.