To provide a fuel injection quantity control device capable of accurately matching the output torque of a diesel engine with the requested torque thereof.

This fuel injection quantity control device having a requested torque computing means 23 for computing the requested torque of the diesel engine 1 to match the output torque with the requested torque is provided with an engine speed detecting means 16 for detecting the engine 1 speed, a fuel injection quantity computing means 20 for computing the fuel injection quantity to the engine 1, a combustion condition detecting means 17 for detecting combustion condition of the engine 1, a memory means 14 for storing a plurality of maps 19 to be used to obtain the output torque on the basis of, at least, the engine speed and the fuel injection quantity in response to the combustion condition, an estimating means 21 for estimating the output torque by selecting the map 19 in response to the combustion condition of the engine 1 and estimating on the basis of the engine speed and the fuel injection quantity, and a correcting means 22 for correcting the fuel injection quantity so that the estimated output torque agrees with the requested torque.