To provide a full close position setting device in a valve actuator capable of setting a full close position of a valve in no relation to a degree of an initial rotation angle of a rotary encoder.

The rotary encoder for measuring a rotation angle of an output shaft is provided and a micro-computer (CPU) capable of detecting an absolute angle of the rotary encoder by a signal from the rotary encoder is loaded in a casing. A measured data memorizing memory (RAM) for memorizing a deviation from a full close angle 0° by closing an origin setting switch when the absolute angle of the rotary encoder is not zero when the valve is positioned at a full close position is provided. The micro-computer (CPU) operates a deviation of the angle preserved in the measured data memorizing memory (RAM) to make it as a reference. An energization instruction is carried out to a motor such that a release angle of the vale becomes the instruction angle.