The engine speed control device (30) executes a first PID gain calculation step wherein a target engine speed Nm is calculated and a first PID gain is calculated on the basis of an engine speed deviation ([Delta]N) between the target engine speed (Nm) and an engine speed (Nr) detected by an engine speed detection means (24), a target rack position calculation step wherein a target rack position (Rset) of a fuel injection pump (2) is calculated by correcting the first PID gain on the basis of a cooling water temperature (Tw) detected by a cooling water temperature detection means (1a), a secondPID gain calculation step wherein a second PID gain is calculated on the basis of a rack position deviation ([Delta]R) between the target rack position (Rset) and a rack position (Rr) detected by a rack position detection means, and a rack control signal production step wherein a rack control signal (Rfset) is produced by correcting the second PID gain on the basis of a lubricating oil temperature (pump oil temperature (Tp)) detected by a lubricating oil temperature detection means, and controls the engine speed by controlling the rack position on the basis of the rack control signal (Rfset).