PURPOSE: To make it possible to precisely detect the opening degree position of a throttle valve which changes due to abrasion, etc. by replacing the stored value of a discriminating variable with an actual valve opening degree when the actual valve opening degree is smaller than the value of the discriminating variable.

CONSTITUTION: Upon operation of an engine, in an ECU5, a first predetermined opening degree value (for example 1.7°) which is larger than a minimum valve opening degree value to which a throttle valve 3 is mechanically limited is stored in memory as the initial value of a discriminating variable, and a second predetermined opening degree value (for example, 0°) which is lower than the above-mentioned minimum valve opening degree value is stored in the memory as the initial value of the full open and close degrees of the throttle valve 3. Then, an actual valve opening degree from a throttle valve opening degree sensor 4 is compared with the discriminating variable value, and if the actual opening degree value is greater than the discriminating variable, the stored value of the discriminating variable value is replaced with the actual valve opening degree at that time. Further when the condition in which the discriminating variable value is equal to an actual vavle opening degree, is sustained for a predetermine period, controll is made such that the above-mentioned full open and close degree values are replaced with the stored value of the discriminating variable.