A method for producing a polymerization-reactive liquid formulation, comprising: a step of feeding a first preparative liquid formulation containing a norbornene-based monomer and a second preparative liquid formulation containing a metathesis polymerization catalyst to a mixing head of a rotary mixer; and a step of mixing the first preparative liquid formulation with the second preparative liquid formulation by the mixing head, wherein the mixing head includes: a casing including a cylindrical mixing chamber closed at one end and open at the other end, and an ejection port disposed at the closed end of the mixing chamber to eject the polymerization-reactive liquid formulation; a cap attached to the open end of the mixing chamber to seal the mixing chamber the cap including a first chemical solution feeding port which feeds the first preparative liquid formulation to the mixing chamber and a second chemical solution feeding port which feeds the second preparative liquid formulation to the mixing chamber; a mixing rotor including a cylindrical rotor body which is accommodated inside the mixing chamber to be rotatable around the central axis of the mixing chamber and extends in the axial direction of the mixing chamber; a connection disposed at one end of the rotor body to be connected to a driving source for the rotary mixer through a connection hole disposed in the cap; and a plurality of protrusions disposed on the circumferential surface of the rotor body to mix the first preparative liquid formulation with the second preparative liquid formulation by rotation of the rotor body; and a bearing member attached to the closed end of the mixing chamber to rotatably support the other end of the rotor body the plurality of protrusions includes first protrusions and second protrusions, the first protrusions having a width in the axial direction of the rotor body larger than that in the circumferential direction thereof, the second protrusions having a width in the axial direction of the rotor body smaller than that in the circumferential direction thereof, and first and second protrusion rows are alternately arranged at a predetermined interval in the circumferential direction of the rotor body, the first protrusion rows being formed of the first protrusions aligned at a predetermined interval in the axial direction of the rotor body, the second protrusion rows being formed of the second protrusions aligned at a predetermined interval in the axial direction of the rotor body. a casing including a cylindrical mixing chamber closed at one end and open at the other end, and an ejection port disposed at the closed end of the mixing chamber to eject the polymerization-reactive liquid formulation; a cap attached to the open end of the mixing chamber to seal the mixing chamber the cap including a first chemical solution feeding port which feeds the first preparative liquid formulation to the mixing chamber and a second chemical solution feeding port which feeds the second preparative liquid formulation to the mixing chamber; a mixing rotor including a cylindrical rotor body which is accommodated inside the mixing chamber to be rotatable around the central axis of the mixing chamber and extends in the axial direction of the mixing chamber; a connection disposed at one end of the rotor body to be connected to a driving source for the rotary mixer through a connection hole disposed in the cap; and a plurality of protrusions disposed on the circumferential surface of the rotor body to mix the first preparative liquid formulation with the second preparative liquid formulation by rotation of the rotor body; and a bearing member attached to the closed end of the mixing chamber to rotatably support the other end of the rotor body the plurality of protrusions includes first protrusions and second protrusions, the first protrusions having a width in the axial direction of the rotor body larger than that in the circumferential direction thereof, the second protrusions having a width in the axial direction of the rotor body smaller than that in the circumferential direction thereof, and first and second protrusion rows are alternately arranged at a predetermined interval in the circumferential direction of the rotor body, the first protrusion rows being formed of the first protrusions aligned at a predetermined interval in the axial direction of the rotor body, the second protrusion rows being formed of the second protrusions aligned at a predetermined interval in the axial direction of the rotor body.