A disk brake system includes a disk, brake pads, and a linear motion actuator including an electric motor, a rotary shaft coupled to the motor through a reduction gear mechanism, planetary rollers formed with helical grooves in their outer surfaces, and an outer ring member having a helical rib engaged in the helical grooves. The outer ring is coupled to one of the brake pads. When the rotary shaft is rotated by the motor, the outer ring member is moved axially through the planetary rollers, and the brake pads are pressed against the disk. The linear motion actuator further includes a locking mechanism including engaging holes formed in an intermediate gear of the reduction gear mechanism at equal intervals, a locking pin, and a linear solenoid for moving the locking pin until the pin engages in one of the engaging holes, thus locking the motor.