Disclosed is a failure detection method of a pressure reducing valve of a brake system. The failure detection method of a pressure reducing valve of a brake system includes: moving, when performing a pressure reducing operation of a brake system, a pump piston by controlling a motor of the brake system; calculating a circuit pressure change amount of the brake system in response to the performing of the pressure reducing operation; and determining that a failure occurs in a pressure reducing valve of the brake system when the circuit pressure change amount does not correspond to a predetermined range.
