A mixing degree adjusting device comprises a pair of spacer members arranged at a position sandwiching both round sectional portions of a pair of screws from both sides in the aligning direction of the pair of screws, and a pair of drawing members arranged as a position sandwiching both the round sectional portions from both sides in the orthogonal direction that is orthogonal to the aligning direction. The spacer members are movable between an adjusting position opposing to the round sectional portions, and a retracting position, when the screws are extracted from the barrel, for retracting to the outside until the passage of the screws is accepted. Opposing portions opposing to the round sectional portions at the time of moving to the adjusting position are formed in a substantially linear shape extending in parallel with the orthogonal direction seen from the axial direction of the screws. The drawing members are movable in the direction approaching to and retracting from the round sectional portions along the opposing portions in an area intervening between the opposing portions of both the spacer members at the time of moving to the adjusting position, and when the screws are extracted from the barrel, retractable to the outside until the passage of the screws is accepted.