A mechanism for opening and closing a hopper gate of the present invention, which is used in a combination weigher which performs combination calculation based on weights of objects, to find a combination in which a total of the weights of the objects falls within an allowable range with respect to a target weight, comprises an actuator (31) for generating a driving force for opening and closing the hopper gate (14A); a transmission mechanism for transmitting the driving force to the hopper gate (14A); a detecting device (60) capable of detecting an original point of a rotary shaft of the transmission mechanism, the original point corresponding to a position in which the hopper gate (14A) is closed; and a controller (18) capable of receiving an output signal of the detecting device (60).