A control device for a diesel engine includes a variable geometry supercharger with a variable supercharge pressure mechanism and an EGR valve configured to adjust an EGR gas amount. In the control device, when the diesel engine is determined to be accelerating, a maximum exhaust pressure is set such that an increased amount of an engine torque by increasing a fuel injection amount in association with acceleration is greater than an increased amount of pumping loss increased with increasing exhaust pressure due to an actuation of the variable supercharge pressure mechanism in association with acceleration. A target control variable for the variable supercharge pressure mechanism and a target control variable for the EGR valve are controlled on the basis of the maximum exhaust pressure.