A ram driving method including the steps of fixing one end of a large-diameter piston rod (5R) integral with a large-diameter piston (5P) reciprocably included in a large-diameter cylinder (5) attached integrally to a ram (3)to a fixing unit (9), connecting one end of a small-diameter piston rod (7R) integral with a small-diameter piston (7P) reciprocably included in a small-diameter cylinder (7) integral with the large-diameter cylinder to a moving member (17) moved by a motor driving, integrally moving the small-diameter cylinder and the small-diameter piston and communicating a first compartment (5A) and a second compartment (5B) of the large-diameter cylinder divided by the large-diameter piston with each other, moving the ram integrally with the small-diameter piston rod moved by the moving member, and communicating the small-diameter cylinder with the large-diameter cylinder, thereby moving the large-diameter cylinder with a strong force by a working fluid supplied from the small-diameter cylinder.