An optical scanning apparatus including: a MEMS mirror that deflects an optical beam output from a light source; a drive unit that drives the MEMS mirror at a constant cycle; and a photodetector device that detects the optical beam output from the light source, a deflection state of the MEMS mirror is obtained from a relationship between a phase of a drive phase signal input to the drive unit and a phase of a detection signal of the photodetector device. For example, time T is detected between output of the drive phase signal and output of the detection signal from the photodetector device, and maximum deflection angle θm of the MEMS mirror is obtained by the equation θm=θ0/sin(2πνT−π/2). T is time, θ0 is an installation deflection angle of the photodetector device, and ν is a deflection frequency of the MEMS mirror.