A fuel supply control device for a bi-fuel internal combustion engine in which a liquid fuel and a gas fuel can be used as a fuel to be used for engine operation, wherein: the fuel supply control device switches the fuel to be used for engine operation from a liquid fuel to a gas fuel, the fuel supply control device judges whether or not a gas fuel can be supplied to a cylinder (#1, #2, #3, #4) serving as a judgment object based on a fuel parameter that fluctuates according to supply of the gas fuel to the cylinder (#1, #2, #3, #4) in a state where only the gas fuel is supplied experimentally to the only one cylinder (#1, #2, #3, #4) serving as a judgment object without supplying with the liquid fuel and the liquid fuel is supplied to other cylinders (#1, #2, #3, #4) when switching the fuel from the liquid fuel to the gas fuel, and the fuel supply control device switches the fuel to be used for engine operation from the liquid fuel to the gas fuel when the fuel supply control device judges that the gas fuel can be supplied to all of the cylinders (#1, #2, #3, #4), wherein when the fuel supply control device judges that there is a cylinder to which the gas fuel cannot be supplied, the fuel supply control device continues the engine operation using the liquid fuel, characterised in that, when the fuel supply control device judges that there is a cylinder (#1, #2, #3, #4) to which the gas fuel cannot be supplied and continues the engine operation using the liquid fuel, the fuel supply control device acquires a viscosity parameter, and the viscosity parameter is a parameter from which the degree of lowering of viscosity of impurities adhering to an injection valve (161, 162, 163, 164) for gas injection can be estimated, and when a re-judgment permission condition based on the viscosity parameter is satisfied, the fuel supply control device performs the judging process again for at least the cylinder (#1, #2, #3, #4) to which it is judged that the gas fuel cannot be supplied of all of the cylinders (#1, #2, #3, #4). the fuel supply control device switches the fuel to be used for engine operation from a liquid fuel to a gas fuel, the fuel supply control device judges whether or not a gas fuel can be supplied to a cylinder (#1, #2, #3, #4) serving as a judgment object based on a fuel parameter that fluctuates according to supply of the gas fuel to the cylinder (#1, #2, #3, #4) in a state where only the gas fuel is supplied experimentally to the only one cylinder (#1, #2, #3, #4) serving as a judgment object without supplying with the liquid fuel and the liquid fuel is supplied to other cylinders (#1, #2, #3, #4) when switching the fuel from the liquid fuel to the gas fuel, and the fuel supply control device switches the fuel to be used for engine operation from the liquid fuel to the gas fuel when the fuel supply control device judges that the gas fuel can be supplied to all of the cylinders (#1, #2, #3, #4), wherein when the fuel supply control device judges that there is a cylinder to which the gas fuel cannot be supplied, the fuel supply control device continues the engine operation using the liquid fuel, characterised in that, when the fuel supply control device judges that there is a cylinder (#1, #2, #3, #4) to which the gas fuel cannot be supplied and continues the engine operation using the liquid fuel, the fuel supply control device acquires a viscosity parameter, and the viscosity parameter is a parameter from which the degree of lowering of viscosity of impurities adhering to an injection valve (161, 162, 163, 164) for gas injection can be estimated, and when a re-judgment permission condition based on the viscosity parameter is satisfied, the fuel supply control device performs the judging process again for at least the cylinder (#1, #2, #3, #4) to which it is judged that the gas fuel cannot be supplied of all of the cylinders (#1, #2, #3, #4).