A gap adjustment mechanism for a disk brake device having a disk rotor provided to be free to rotate about a predetermined axis and to be free to slide in a direction of the predetermined axis, a first friction pad disposed on one surface side of the disk rotor in the predetermined axis direction with a first gap, a second friction pad disposed on another surface side of the disk rotor in the predetermined axis direction with a second gap, and pad driving means for pushing the first friction pad in the predetermined axis direction, the disk brake device generating a braking force by operating the pad driving means to move the first friction pad to the disk rotor side such that the first friction pad pushes the disk rotor in the predetermined axis direction while contacting the disk rotor until the disk rotor comes into contact with the second friction pad, whereby the disk rotor and the friction pads (11, 12) are frictionally engaged, - the gap adjustment mechanism comprising: - first and second tappet members (33, 76) provided to be free to rotate respectively in only one rotation direction when the pad driving means is operated; - a first piston member that is screwed to the first tappet member, provided in contact with the first friction pad so as to be incapable of rotating, and moved to the disk rotor side relative to the first tappet member when the first tappet member rotates; - a second piston member that is screwed to the second tappet member, provided in contact with the second friction pad so as to be incapable of rotating, and moved to the disk rotor side relative to the second tappet member when the second tappet member rotates; - a first adjustment mechanism that adjusts a gap formed between the first friction pad and the disk rotor due to wear on the first friction pad; and - a second adjustment mechanism that adjusts a gap formed between the second friction pad and the disk rotor due to wear on the second friction pad, characterized in that, - the first adjustment mechanism is adapted to displace the first piston member to the disk rotor side relative to the first tappet member until the first friction pad comes into contact with the disk rotor at a predetermined surface pressure after being displaced to the disk rotor side by an amount corresponding to the first gap, and that - the second adjustment mechanism is adapted to displace the second piston member to the disk rotor side relative to the second tappet member until the disk rotor comes into contact with the second friction pad at a predetermined surface pressure after being displaced to the second friction pad side by an amount corresponding to the second gap. - the gap adjustment mechanism comprising: - first and second tappet members (33, 76) provided to be free to rotate respectively in only one rotation direction when the pad driving means is operated; - a first piston member that is screwed to the first tappet member, provided in contact with the first friction pad so as to be incapable of rotating, and moved to the disk rotor side relative to the first tappet member when the first tappet member rotates; - a second piston member that is screwed to the second tappet member, provided in contact with the second friction pad so as to be incapable of rotating, and moved to the disk rotor side relative to the second tappet member when the second tappet member rotates; - a first adjustment mechanism that adjusts a gap formed between the first friction pad and the disk rotor due to wear on the first friction pad; and - a second adjustment mechanism that adjusts a gap formed between the second friction pad and the disk rotor due to wear on the second friction pad, characterized in that, - the first adjustment mechanism is adapted to displace the first piston member to the disk rotor side relative to the first tappet member until the first friction pad comes into contact with the disk rotor at a predetermined surface pressure after being displaced to the disk rotor side by an amount corresponding to the first gap, and that - the second adjustment mechanism is adapted to displace the second piston member to the disk rotor side relative to the second tappet member until the disk rotor comes into contact with the second friction pad at a predetermined surface pressure after being displaced to the second friction pad side by an amount corresponding to the second gap.