The purpose of the present invention is to provide an upright piano action mechanism that improves sound quality relating to dynamic range and the like and facilitates repetition of the same note on the keyboard of the upright piano and any form of key pressing such as repetition and play-ing at half-touch while playing at pianissimo and half touch with the damper pedal depressed. The action mechanism is configured to either reinforce the spring force of a single hammer butt spring or use two hammer butt springs. When using two hammer butt springs, a butt range cord is provided with a twisted part to cause the hook of the two hammer butt springs to be caught by a ring-shaped part at the tip of the twisted part and the twisted part. The action mechanism uses a material with excellent lubricity for a hammer butt skin while reinforcing the elasticity of a jack spring, and is provided with a rubber-based cushion having a protrusion made of an elastic material with a through-hole along the rear surface of a jack stop rail and the front and rear surfaces of a damper stop rail.