A fuel supply controlling device for use in a divided-chamber gas engine, comprising: an auxiliary chamber fuel supply valve configured to supply a gaseous fuel to an auxiliary chamber during an auxiliary chamber fuel supply valve-open duration, and to stop supplying the gaseous fuel to the auxiliary chamber during an auxiliary chamber fuel supply valve-closed duration; a non-return valve interposed between the auxiliary chamber fuel supply valve and the auxiliary chamber, the non-return valve being configured to block a reverse flow from the auxiliary chamber in a valve-closed state, to become a valve-opened state as a result of a valving element of the non-return valve being moved by pressure of the gaseous fuel when the auxiliary chamber fuel supply valve is opened, and in the valve-opened state, to allow a flow of the gaseous fuel from the auxiliary chamber fuel supply valve into the auxiliary chamber; a valve state detector configured to detect an operating state of the non-return valve; a rotation angle detector configured to detect at least one of a rotation angle of a crank shaft or a rotation angle of a cam shaft; and a controller configured to determine an operation command value of the auxiliary chamber fuel supply valve, wherein the controller measures an actual operating state of the non-return valve based on signals from the valve state detector and the rotation angle detector, the actual operating state being associated with the detected rotation angle, and corrects the operation command value of the auxiliary chamber fuel supply valve such that the actual operating state is brought close to a target operating state. an auxiliary chamber fuel supply valve configured to supply a gaseous fuel to an auxiliary chamber during an auxiliary chamber fuel supply valve-open duration, and to stop supplying the gaseous fuel to the auxiliary chamber during an auxiliary chamber fuel supply valve-closed duration; a non-return valve interposed between the auxiliary chamber fuel supply valve and the auxiliary chamber, the non-return valve being configured to block a reverse flow from the auxiliary chamber in a valve-closed state, to become a valve-opened state as a result of a valving element of the non-return valve being moved by pressure of the gaseous fuel when the auxiliary chamber fuel supply valve is opened, and in the valve-opened state, to allow a flow of the gaseous fuel from the auxiliary chamber fuel supply valve into the auxiliary chamber; a valve state detector configured to detect an operating state of the non-return valve; a rotation angle detector configured to detect at least one of a rotation angle of a crank shaft or a rotation angle of a cam shaft; and a controller configured to determine an operation command value of the auxiliary chamber fuel supply valve, wherein the controller measures an actual operating state of the non-return valve based on signals from the valve state detector and the rotation angle detector, the actual operating state being associated with the detected rotation angle, and corrects the operation command value of the auxiliary chamber fuel supply valve such that the actual operating state is brought close to a target operating state.