PURPOSE: To maintain the optimum fuel injection quantity in response to the fuel used in a Diesel engine.

CONSTITUTION: The fuel injection quantity control device of a Diesel engine is provided with a means 1 setting the basic injection quantity Qn for the reference fuel in response to the load of the engine, a means 2 detecting the viscosity of the fuel being used actually, a means 3 judging whether the viscosity of the fuel being used is lower than that of the reference fuel or not, a means 4 augmentatively correcting the basic injection quantity Qn when the viscosity of the fuel being used is lower than that of the reference fuel based on the judged result, and a means 5 controlling an injection quantity adjusting mechanism 6 to obtain the augmentatively corrected injection quantity Q.