A throttle device for an internal-combustion engine, in which, on one surface of a throttle body side wall is formed a mounting space for mounting a reduction gear mechanism which transmits the power of a motor to a throttle valve shaft; and a throttle sensor for detecting the throttle valve opening is built inside of the gear cover covering the mounting space, and is covered with a sensor cover. A shaft hole of a rotor of the throttle sensor is exposed out through the sensor cover. When the gear cover is attached to the side wall of the throttle body, one end of the throttle valve shaft fits in the rotor shaft hole by elastically deforming a fitting spring inserted in the shaft hole, thereby enabling downsizing, weight reduction, and simplification of assembly and wiring harness of the electronically controlled throttle device, and realization of stabilized operation and improved accuracy of the throttle sensor.