A master cylinder having at least one piston that pressurizes a hydraulic fluid when a stepping force on a brake pedal is transmitted thereto and sends the pressurized hydraulic fluid to at least a wheel cylinder. The piston includes a large diameter passage and a small diameter passage having a diameter smaller than that of the large diameter passage, and a valve body is fastened to the large diameter passage to control fluid communication between the large diameter passage and the small diameter passage, the valve body including a plunger coupling hole and a channel through which the hydraulic fluid communicates between the large diameter passage and the small diameter passage, wherein a plunger slidably passes the valve body through the plunger coupling hole and closes the channel of the valve body when the stepping force on the brake pedal is transmitted to the piston.