The invention provides a rotational angle detection device and electric power steering system. An ECU (11) detects a steering angle ([theta]s) that is an absolute angle based on a sine signal (S_sin) and a cosine signal (S_cos) that are output from a steering sensor. The ECU (11) includes a three-phase pulse generator (30) that generates, based on the sine signal (S_sin) and the cosine signal (S_cos), three-phase pulse signals (P1, P2, P3) that have edges corresponding to predetermined rotational angles and that are set in such a manner that the predetermined rotational angles corresponding to the respective edges do not overlap each other and are at regular intervals. A microcomputer (21) has a function as a second rotational angle detector that calculates a steering angle based on the pulse signals(P1, P2, P3).