To provide technology for inferring gas temperature in a cylinder accurately without requiring many matching processes irrespective of change of engine specifications by putting physical phenomenon related to bahavior of intake gas in a suction system of an engine including the inside of the cylinder into a model.

This intake gas temperature inferring device is provided with a means A01 for obtaining gas temperature before thermal insulation change of a section in which gas in the suction system is compressed or expanded by thermal insulation, a means A02 for obtaining gas pressure before and after thermal insulation change in the section, a means A03 for obtaining specific heat ratio of gas in the section, and a gas temperature computing means A04 for inferring gas temperature in the suction system using results of each means. The phenomenon in which intake gas changes gas temperature by thermal insulation compression/expansion generated in the suction system in accordance with opening and closing condition of a suction throttle valve and load fluctuation is put into the model. By this model, adaptation on desk becomes possible individually per change section in accordance with the model irrespective of change of control characteristics and specifications of a part affecting thermal insulation change of intake gas such as a throttle chamber and a suction valve.