A brake device for a vehicle comprising a master cylinder that generates a brake fluid pressure by operation of a brake pedal, a stroke simulator that imparts a reaction force to the operation of the brake pedal, a plurality of wheel cylinders (26, 27, 30, 31) each of which brakes a wheel, a master cut valve (32, 33) that is capable of blocking a fluid path providing a connection between the master cylinder and the wheel cylinders (26, 27, 30, 31), a slave cylinder that is disposed between the master cut valve (32, 33) and the wheel cylinders (26, 27, 30, 31) and generates a brake fluid pressure that is commensurate with the operation of the brake pedal, and control means that, when the brake pedal is operated, closes the master cut valve (32, 33) and actuates the slave cylinder in a state in which the stroke simulator is capable of being actuated, in which the slave cylinder includes: a cylinder main body; first and second pistons (48A, 48B) that are slidably fitted into the cylinder main body and urged by springs in respective return directions; a first fluid pressure chamber that is adjacent to a front surface of the first piston and communicates with the wheel cylinders (26, 27) of a first line; a second fluid pressure chamber that is adjacent to a front surface of the second piston and communicates with the wheel cylinders (30, 31) of a second line; and a motor that drives the first and second pistons (48A, 48B) forward when operated to rotate, and the motor is operated to rotate in accordance with a depressing operation of the brake pedal so as to change a stroke of the slave cylinder, characterized in that the brake device further comprises idling detection means that detects idling of a the motor of the slave cylinder, wherein when the stroke of the slave cylinder in a braking process is equal to or above a predetermined stroke in which even if one of the first and second fluid pressure chambers (50A, 50B) cannot generate a brake fluid pressure, the other can generate it, the idling detection means detects idling of the motor, and when idling of the motor is detected, the control means opens the master cut valve (32, 33) and actuates the wheel cylinders (26, 27, 30, 31) by means of brake fluid pressure generated by the master cylinder. a cylinder main body; first and second pistons (48A, 48B) that are slidably fitted into the cylinder main body and urged by springs in respective return directions; a first fluid pressure chamber that is adjacent to a front surface of the first piston and communicates with the wheel cylinders (26, 27) of a first line; a second fluid pressure chamber that is adjacent to a front surface of the second piston and communicates with the wheel cylinders (30, 31) of a second line; and a motor that drives the first and second pistons (48A, 48B) forward when operated to rotate, and the motor is operated to rotate in accordance with a depressing operation of the brake pedal so as to change a stroke of the slave cylinder, characterized in that the brake device further comprises idling detection means that detects idling of a the motor of the slave cylinder, wherein when the stroke of the slave cylinder in a braking process is equal to or above a predetermined stroke in which even if one of the first and second fluid pressure chambers (50A, 50B) cannot generate a brake fluid pressure, the other can generate it, the idling detection means detects idling of the motor, and when idling of the motor is detected, the control means opens the master cut valve (32, 33) and actuates the wheel cylinders (26, 27, 30, 31) by means of brake fluid pressure generated by the master cylinder.