A trigger device is mounted on a rivet gun. The rivet gun has a barrel, a handle and a pneumatic cylinder. The trigger device has an inlet channel defined in the handle, an outlet hole defined through the handle and communicating with the barrel, a valve-driving channel defined in the handle and communicating with the pneumatic cylinder, a space defined in the handle and communicating with the inlet channel, outlet channel and valve-driving channel. A trigger member is mounted on the handle. A two-step pressing assembly is mounted in the space and may be activated to control the rivet gun to eject a core pin of a used rivet or rivet pieces with an unused rivet. High-pressure air from an air source connected to the rivet gun is output only during the ejection operation of the rivet gun to prevent unnecessary air consumption and noise.