A fuel supply controlling device (100) comprises a prechamber fuel supply valve (18) that supplies gas fuel to a prechamber (24), a check valve (19) that is disposed between the prechamber fuel supply valve (18) and the prechamber (24) and prevents back flow from the prechamber (24), a valve state detecting device (51) that detects the operational state of the check valve (19), a rotational angle detecting device (56) that detects the rotational angle in the engine cycle, and a controlling device (60) that determines the operational command value of the prechamber fuel supply valve (18). The controlling device (60) measures, on the basis of a signal from the valve state detecting device (51) and the rotational angle detecting device (56), the actual operating state of the check valve (19) and by associating the same to a rotational angle, and corrects the operational command value for the prechamber fuel supply valve (18) so that the measured actual operating state approaches the target operating state.