A servo drive system of a press machine includes a ram, an operation shaft which vertically moves the ram, and a servo motor which operates as a power source of the ram and which generates ram pressure using a torque based on at least one speed-torque characteristic of the servo motor. The operation shaft is directly driven using the servo motor. The at least one speed-torque characteristic of the servo motor is set in a manner such that a motor torque of the servo motor at which a capacity of electric energy supplied by a servo amplifier is determined becomes the motor torque at which an optimal punching pattern of the ram is generated from a light load to a heavy load according to a type of workpiece to be worked by the press machine.