A brake system for a vehicle including two hydraulic systems of an input side hydraulic system (Pin) and an output side hydraulic system (Pout), each of the two hydraulic systems being capable of generating a brake fluid pressure on a brake fluid, comprising: an input device to generate a first brake fluid pressure in the input side hydraulic system according to an operational amount of an operation piece, the input device being disposed in the input side hydraulic system (Pin); a fluid pressure generating device comprising an actuator (72, 74) to generate a second brake fluid pressure in the output side hydraulic system according to the operational amount of the operation piece, the fluid pressure generating device being disposed in the output side hydraulic system (Pout); a master-cut valve (60a, 60b) to selectively flow the brake fluid between the input side hydraulic system (Pin) and the output side hydraulic system (Pout) when opened, and to shut off flowing of the brake fluid between the input side hydraulic system (Pin) and the output side hydraulic system when closed; an out-valve (128, 130) through which the brake fluid can flow into a reservoir unit when opened; and a control unit which, when the operation piece is operated, closes the master-cut valve and the out-valve and controls the actuator (72, 74) to generate the second brake fluid pressure according to a sensed value of the operational amount of the operation piece, characterized in that the control unit opens one or both of the master-cut valve (60a, 60b) and the out-valve (128, 130) when the control unit determines that an applied output characteristic of the second brake fluid pressure, according to an operational amount (SSt) of the actuator (72, 74), shifts from a basic output characteristic (Tstd) set beforehand to a changed output characteristic (Thigh), in which a minute brake fluid pressure (ΔP0) occurs in the output side hydraulic system when the operational amount (SSt) of the actuator (72, 74) is zero. an input device to generate a first brake fluid pressure in the input side hydraulic system according to an operational amount of an operation piece, the input device being disposed in the input side hydraulic system (Pin); a fluid pressure generating device comprising an actuator (72, 74) to generate a second brake fluid pressure in the output side hydraulic system according to the operational amount of the operation piece, the fluid pressure generating device being disposed in the output side hydraulic system (Pout); a master-cut valve (60a, 60b) to selectively flow the brake fluid between the input side hydraulic system (Pin) and the output side hydraulic system (Pout) when opened, and to shut off flowing of the brake fluid between the input side hydraulic system (Pin) and the output side hydraulic system when closed; an out-valve (128, 130) through which the brake fluid can flow into a reservoir unit when opened; and a control unit which, when the operation piece is operated, closes the master-cut valve and the out-valve and controls the actuator (72, 74) to generate the second brake fluid pressure according to a sensed value of the operational amount of the operation piece, characterized in that the control unit opens one or both of the master-cut valve (60a, 60b) and the out-valve (128, 130) when the control unit determines that an applied output characteristic of the second brake fluid pressure, according to an operational amount (SSt) of the actuator (72, 74), shifts from a basic output characteristic (Tstd) set beforehand to a changed output characteristic (Thigh), in which a minute brake fluid pressure (ΔP0) occurs in the output side hydraulic system when the operational amount (SSt) of the actuator (72, 74) is zero. the control unit opens one or both of the master-cut valve (60a, 60b) and the out-valve (128, 130) when the control unit determines that an applied output characteristic of the second brake fluid pressure, according to an operational amount (SSt) of the actuator (72, 74), shifts from a basic output characteristic (Tstd) set beforehand to a changed output characteristic (Thigh), in which a minute brake fluid pressure (ΔP0) occurs in the output side hydraulic system when the operational amount (SSt) of the actuator (72, 74) is zero.