To provide a rotation angle detection device capable of reducing the maximum rotation angle of a rotator for detection an abnormality generated when switching off a power source after switching on the power source.

A microcomputer detects each rotation angle of the first and second driven gears respectively engaged with a main driving gear rotating integrally with a steering shaft. Each radius value r1, r2 is calculated based on each analog signal outputted from the first and second magnetometric sensors. The ratio r1/r2 of the radius values r1, r2 is calculated, and relative radius check for comparing the ratio r1/r2 of the radius values with a lower limit threshold RL and an upper limit threshold RH set beforehand is performed.

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