The present invention is intended to suppress an excessive increase in the amount of emission of HC in a multi-fuel internal combustion engine of compression ignition type which is able to perform mixed combustion of a liquid fuel, which can be ignited by compression, and a gas fuel which has methane as a primary component. In the present invention, a required mixing ratio of the liquid fuel and the gas fuel as well as a required amount of HC emission is calculated based on an operating state of the multi-fuel internal combustion engine (S103, S104). Then, based on the required mixing ratio and the required amount of HC emission, a required compression ratio is calculated which is a compression ratio at which an amount of HC emission from the multi-fuel internal combustion engine becomes the required amount of HC emission (S105), and the compression ratio of the multi-fuel internal combustion engine is controlled to the required compression ratio (S107).