A parking brake unit (101; 201; 301) of a brake device (100; 200; 300) for braking a wheel as an output member of an actuator presses a brake lining against a brake disk rotating with the wheel by supply of hydraulic fluid, the parking brake unit (101; 201; 301) comprising: a first piston (110; 210) formed in a cylindrical shape, the first piston (110; 210) having a first engaging part on an inner circumference thereof, the first piston (110; 210) being configured to move in first direction by pressure of the hydraulic fluid supplied to a pressure chamber defined in the back thereof; a first biasing member configured to bias the first piston (110; 210) in second direction, opposite to the first direction; a second piston (120; 220; 320) slidably provided on the inner circumference of the first piston (110; 210), the second piston (120; 220; 320) having a second engaging part on an outer circumference thereof; an engaging member configured to cause the first engaging part and the second engaging part to engage with each other; and wherein the second piston (120; 220; 320) causes the brake lining to move in direction bringing the wheel to be in a braking state, when the first piston (110; 210) moves in the second direction characterised by a releasing member slidably provided on an inner circumference of the second piston (120; 220; 320), the releasing member having a third engaging part on an outer circumference thereof, wherein the second engaging part is formed to penetrate the second piston (120; 220; 320) in radial direction, and wherein the engaging member causes the second engaging part to engage with the third engaging part so that the engagement between the first engaging part and the second engaging part is released, when releasing operation is made. a first piston (110; 210) formed in a cylindrical shape, the first piston (110; 210) having a first engaging part on an inner circumference thereof, the first piston (110; 210) being configured to move in first direction by pressure of the hydraulic fluid supplied to a pressure chamber defined in the back thereof; a first biasing member configured to bias the first piston (110; 210) in second direction, opposite to the first direction; a second piston (120; 220; 320) slidably provided on the inner circumference of the first piston (110; 210), the second piston (120; 220; 320) having a second engaging part on an outer circumference thereof; an engaging member configured to cause the first engaging part and the second engaging part to engage with each other; and wherein the second piston (120; 220; 320) causes the brake lining to move in direction bringing the wheel to be in a braking state, when the first piston (110; 210) moves in the second direction characterised by a releasing member slidably provided on an inner circumference of the second piston (120; 220; 320), the releasing member having a third engaging part on an outer circumference thereof, wherein the second engaging part is formed to penetrate the second piston (120; 220; 320) in radial direction, and wherein the engaging member causes the second engaging part to engage with the third engaging part so that the engagement between the first engaging part and the second engaging part is released, when releasing operation is made.