A vehicle brake system includes a cylinder, a master piston including a pressure applying piston and a projection portion, a servo chamber, a contact/separation determining means determining a separated state and a contact state between the input piston and the master piston, a pilot pressure generating device generating a pilot pressure, a servo pressure generating device, a servo pressure measuring device measuring a servo pressure, and a master pressure estimating means estimating a master pressure from the pilot pressure and a first servo ratio, which is a cross-sectional area ratio between a first pilot chamber and a servo pressure generating chamber, in a case of the separated state, and estimating the master pressure based on the servo pressure, the pilot pressure and a second servo ratio, which is a cross-sectional area ratio between a second pilot chamber and the servo pressure generating chamber, in a case of the contact state.