The present invention provides a magnetic rotational-angle detector capable of detecting a position and a rotational angle of a rotating member with excellent accuracy. The magnetic rotational-angle detector includes device arrays (51, 52) constituted by a plurality of magnetic sensor devices which are arranged to cancel the n-th order harmonic components by each other. In the device arrays, the magnetic sensor devices corresponding to each other are arranged such that they are spaced apart with a distance of (2m+1)λ, and the arrangement of all the magnetic sensor devices and their orientations with respect to a power-supply terminal and a ground terminal are axisymmetrical about a straight line (2b) passing through a barycenric position (59) of all the magnetic sensor devices which is positioned on a straight line along a radial direction of a rotating drum (1).