A control device divides 360° corresponding to one cycle of a resolver angle into N zones, and determines whether or not a resolver angle θ in the current cycle exceeds a division border. When determined that resolver angle θ in the current cycle exceeds a division border, the control device calculates a time difference ΔT[n] between a calculation time T[n] in the immediately preceding resolver cycle and a calculation time T in the current cycle. The control device also calculates a resolver angle variation Δθ[n] with time difference ΔT[n] by adding 360° to the difference between resolver angle θ obtained in the current cycle and a resolver angle θ[n] obtained in the immediately preceding resolver cycle. The control device then calculates a rotation speed NM by multiplying, by a coefficient K, a value obtained by dividing Δθ[n] by ΔT[n].