Abstract: A mixing apparatus having a mixing chamber which has inlet ports of resin components to be mixed and an outlet port of a mixture thus obtained, comprising a flow varying member which moves across the mixing chamber to selectively occupy a retracted position in which it is retracted from the mixing chamber and a forward position in which the flow varying member interrupts the flow of the resin components to change the directions thereof, an immovable key member which defines a restriction passage together with the flow varying member which occupies the forward position, and a chamber block which is opposed to the flow varying member to laterally move in order to selectively occupy a forward position in which it defines a part of the mixing chamber and a retracted position in which the chamber block defines a flow varying chamber together with the key member and the flow varying member which occupies the forward position.