A vehicle brake system installed in a vehicle comprising: an atmospheric pressure detecting device that detects atmospheric pressure to output a detected signal of atmospheric pressure (P air); a negative pressure booster that uses negative pressure to boost a brake operation force inputted via a brake operation mechanism; a negative pressure generating device that supplies the negative pressure to the negative pressure booster; a booster pressure detecting device that detects booster pressure generated in the negative pressure booster to output a detected signal of booster pressure; an activation determining unit that compares a predetermined activation threshold value (PthON) to be set to have larger negative pressure as the atmospheric pressure (P air) increases with a determination value calculated based on the detected signal of booster pressure to determine activation of the negative pressure generating device; and a deactivation determining unit that compares a predetermined deactivation threshold value (PthOFF) to be set to have larger negative pressure as the atmospheric pressure increases with the determination value to determine deactivation of the negative pressure generating device, characterized in that the activation threshold value (PthON) has a maximum value of negative pressure detected by the booster pressure detecting device in a region where the atmospheric pressure (P air) is greater than or equal to a predetermined high-pressure boundary value (paH), and the activation threshold value (PthON) has a minimum value of negative pressure detected by the booster pressure detecting device in a region where the atmospheric pressure (P air) is less than or equal to a predetermined low-pressure boundary value (paL). an atmospheric pressure detecting device that detects atmospheric pressure to output a detected signal of atmospheric pressure (P air); a negative pressure booster that uses negative pressure to boost a brake operation force inputted via a brake operation mechanism; a negative pressure generating device that supplies the negative pressure to the negative pressure booster; a booster pressure detecting device that detects booster pressure generated in the negative pressure booster to output a detected signal of booster pressure; an activation determining unit that compares a predetermined activation threshold value (PthON) to be set to have larger negative pressure as the atmospheric pressure (P air) increases with a determination value calculated based on the detected signal of booster pressure to determine activation of the negative pressure generating device; and a deactivation determining unit that compares a predetermined deactivation threshold value (PthOFF) to be set to have larger negative pressure as the atmospheric pressure increases with the determination value to determine deactivation of the negative pressure generating device, characterized in that the activation threshold value (PthON) has a maximum value of negative pressure detected by the booster pressure detecting device in a region where the atmospheric pressure (P air) is greater than or equal to a predetermined high-pressure boundary value (paH), and the activation threshold value (PthON) has a minimum value of negative pressure detected by the booster pressure detecting device in a region where the atmospheric pressure (P air) is less than or equal to a predetermined low-pressure boundary value (paL).