A mixer includes: a casing having an introduction port at an upper portion and having a rotor provided with a blade portion for mixing a material to be mixed, inside thereof; a floating weight which blocks the introduction port; and a floating weight moving mechanism which has a horizontal movement unit capable of horizontally moving the floating weight in a supported state between a delivery position just above the introduction port and a retracted position being lateral to the introduction port, on the upper side of the introduction port, and can move the floating weight separately from the horizontal movement unit from the delivery position to a blocking position at which the floating weight blocks the introduction port.