A rotational position sensor having a compact and simple structure, in which a magnetic surface can be formed to have a more accurate shape, and an electronically controlled throttle device for an internal combustion engine, which employs the rotational position sensor. The rotational position sensor comprises a pair of substantially half disk-shaped magnetic cores disposed in an opposed relation to each other with a magnetic sensitive device interposed therebetween, a pair of substantially arc-shaped magnetic cores disposed in an opposed relation to each other along outer peripheries of the half disk-shaped magnetic cores, and a permanent magnet fixed in contact with at least one of the arc-shaped magnetic cores. Magnetic flux generating from the permanent magnet flows successively through one of the arc-shaped magnetic cores, the half disk-shaped magnetic cores, and the other of the arc-shaped magnetic cores, followed by returning to the permanent magnet. The half disk-shaped magnetic cores or the permanent magnet is rotatable, and the magnetic sensitive device detects an amount of the magnetic flux varying with rotation of the half disk-shaped magnetic cores or the permanent magnet.