A brake system comprising: a master cylinder configured to generate a brake hydraulic pressure in response to an operation on a brake operator; a first solenoid valve provided between the master cylinder and a wheel brake (FL, RR, RL, FR); a stroke simulator provided between the master cylinder and the first solenoid valve and connected to the master cylinder through a second solenoid valve; a slave cylinder apparatus including an electric actuator, and a cylinder mechanism configured to generate a brake hydraulic pressure, which is applied to the wheel brake (FL, RR, RL, FR), by a driving force from the electric actuator; an operation amount detector configured to detect the operation amount of the brake operator; a control apparatus configured to control the first solenoid valve, the second solenoid valve and the electric actuator, wherein the control apparatus includes a storage configured to store a reference table indicating a relationship between the operation amount and a reference deceleration, wherein the control apparatus is configured to close the first solenoid valve and open the second solenoid valve when the brake hydraulic pressure is increased by the electric actuator, and the control apparatus is configured to calculate the reference deceleration based on the operation amount detected by the operation amount detector and the reference table, characterized in that: the control apparatus further includes a deceleration estimator configured to estimate a deceleration of the vehicle, and the control apparatus is further configured to control the electric actuator to increase the brake hydraulic pressure so that a difference between the deceleration estimated by the deceleration estimator and the reference deceleration is reduced when the estimated deceleration is smaller than the reference deceleration. a master cylinder configured to generate a brake hydraulic pressure in response to an operation on a brake operator; a first solenoid valve provided between the master cylinder and a wheel brake (FL, RR, RL, FR); a stroke simulator provided between the master cylinder and the first solenoid valve and connected to the master cylinder through a second solenoid valve; a slave cylinder apparatus including an electric actuator, and a cylinder mechanism configured to generate a brake hydraulic pressure, which is applied to the wheel brake (FL, RR, RL, FR), by a driving force from the electric actuator; an electric actuator, and a cylinder mechanism configured to generate a brake hydraulic pressure, which is applied to the wheel brake (FL, RR, RL, FR), by a driving force from the electric actuator; an operation amount detector configured to detect the operation amount of the brake operator; a control apparatus configured to control the first solenoid valve, the second solenoid valve and the electric actuator, wherein the control apparatus includes a storage configured to store a reference table indicating a relationship between the operation amount and a reference deceleration, wherein the control apparatus is configured to close the first solenoid valve and open the second solenoid valve when the brake hydraulic pressure is increased by the electric actuator, and the control apparatus is configured to calculate the reference deceleration based on the operation amount detected by the operation amount detector and the reference table, characterized in that: the control apparatus further includes a deceleration estimator configured to estimate a deceleration of the vehicle, and the control apparatus is further configured to control the electric actuator to increase the brake hydraulic pressure so that a difference between the deceleration estimated by the deceleration estimator and the reference deceleration is reduced when the estimated deceleration is smaller than the reference deceleration.