An apparatus for detecting the opening of the throttle valve of an internal combustion engine. The apparatus comprises a throttle body constituting a portion of a suction passage, a valve shaft held to the throttle body, a throttle valve for controlling the flow of air inside the suction passage, and a sensor fixed to the throttle body. The sensor has a driving shaft one end of which is disposed outside the case of the sensor. A U-shaped engaging member having two legs is fixed to this one end of the driving shaft. A disklike engaging member having two grooves is fixed to one end of the valve shaft. The two legs are fitted in the two grooves.