A gas mixer includes at least one outlet pipe and a base having an inlet portion and a mixing portion. The inlet portion has an air inlet, at least one gas inlet, and an air path. The air path communicates with the air inlet and the at least one gas inlet, and has an exit. The mixing portion has a mixing chamber communicating with the exit. The outlet pipe is engaged with the mixing portion of the base, and extends into the mixing chamber. The exit of the air path corresponds to a body of the outlet pipe. Therefore, air and gas can be effectively premixed and outputted to a burner, whereby to accurately control the air-fuel ratio to enhance the combustion efficiency of the burner.