A target engine torque after an environmental correction is calculated by interpolating it between an environmentally corrected maximum engine torque and an environmentally corrected minimum engine torque such that a ratio of a nominal target engine torque between a nominal maximum engine torque and a nominal minimum engine torque under a predetermined environmental condition becomes essentially equal to a ratio of the target engine torque between the environmentally corrected maximum engine torque and the environmentally corrected minimum engine torque. The environmentally corrected maximum engine torque is obtained by multiplying the nominal maximum engine torque and a correction coefficient according to the environmental condition together.