PURPOSE: To provide a throttle valve opening/closing state detecting device for controlling an engine which is constituted to eliminate a need for a throttle opening sensor and an idle switch and reduce a cost.

CONSTITUTION: A throttle valve opening/closing state detecting device for controlling an engine comprises a pressure valve computing means 20 to compute first and second pressure values related to a pressure in an intake pipe 3 according to the number of revolutions of an engine when a throttle valve 4 in an intake pipe 3 is in a closed state; and a parameter computing means 20 to compute a parameter by means of which a relation of an intake pipe pressure Pb with first and second pressure values is indicated. The throttle valve opening closing state detecting device comprises a learning value computing means 20 to store a representative value, obtained by computing and processing the parameter in time series, as a learning value; a pressure estimated value computing means 20 to compute the pressure estimated value of the intake pipe based on a learning value and first and second values when a throttle valve is in a closed state; and a throttle valve closing state detecting means 20 to decide that the throttle valve is in a closed state when an intake pipe pressure is below a pressure estimated value in a given relation.