The invention minimizes the man-hours required to develop a map that estimates the torque of an internal combustion engine. The method according to the invention includes making a torque estimation model that defines the relation between indicated torque and characteristic values that represent the flow of gas and the combustion state in an internal combustion engine; determining the value of a parameter that relates to a heat-generation rate (dQ/dθ), which is the rate of change in a heating value Q in a cylinder with respect to the crank angle θ, based on an operating condition of the engine; calculating the heat-generation rate under the desired operating condition; estimating the indicated torque of the internal combustion engine with the torque estimation model, using the heat-generation rate. The method according to the invention results in more accurate control over the operating state of an internal combustion engine.