To provide a rotational angle detecting device which can detect the rotational angle of a steering wheel for automobile, etc., with accuracy without receiving any influence from the ambient temperature.

A moving steel piece 36 is fitted to a steering column 12 which journals a steering wheel and a E-type differential transformer (detecting section 28) is installed at the position at which the transformer is counterposed to the steel piece 36 when the steering wheel is maintained at the position at which the automobile steered with the steering wheel goes straight on. When the steering wheel is turned, the column 12 rotates and the amplitude value of the differential voltage outputted across the terminals A and B of the differentially connected secondary coils 32B and 32C of the detecting section 28 becomes proportional to the rotational angle of the column 12. Therefore, the rotational angle of the steering wheel can be detected from the amplitude value of the differential voltage. In addition, when a rotational angle detecting device is constituted in such a way, the device has a good temperature characteristic and does not rely upon the ambient temperature.