A rotation angle detecting device includes a resolver that outputs three-phase signals based on a rotation angle of a rotary shaft, and an R/D converter that acquires the three-phase output signals via signal lines corresponding to the respective phases. The R/D converter determines the rotation angle of the rotary shaft on the basis of the three-phase output signals. The R/D converter includes switching elements that change the potential of the first-phase signal line and the potential of the second-phase signal line, respectively.