A method for adjusting the viscosity of a polypropylene by mixing the polypropylene and peroxide, comprising: preparing a mixing extruder including a barrel, a screw rotatably arranged in the barrel and having a plurality of mixing wings including a first mixing wing located at a most upstream side and a second mixing wing located downstream of the first mixing wing, peroxide feeder for feeding peroxide at a position upstream of the first mixing wing, a first flow path throttling mechanism capable of changing an area of a flow path for the polypropylene at a side downstream of the first mixing wing and upstream of the second mixing wing to thereby change a filling ratio of a polypropylene mixed by the first mixing wing, and a second flow path throttling mechanism capable of changing the area of the flow path for the polypropylene at the downstream side of the second mixing wing to thereby change a filling ratio of the polypropylene mixed by the second mixing wing; and mixing of the polypropylene and peroxide having a half-life equal to or longer than 10 seconds at 200°C using this mixing extruder, wherein, in the mixing, the flow path for the polypropylene located downstream of the first mixing wing is throttled by the first flow path throttling mechanism to thereby increase the filling ratio of the polypropylene at the first mixing wing; and, when the temperature of the polypropylene mixed by the most downstream one of the mixing wings (21, 22) is below a predetermined temperature, the flow path for the polypropylene located downstream of the second mixing wing is throttled by the second flow path throttling mechanism to increase the filling ratio of the polypropylene at the second mixing wing, thereby bringing the polypropylene mixed by the mixing wing at the most downstream side to or above the predetermined temperature. preparing a mixing extruder including a barrel, a screw rotatably arranged in the barrel and having a plurality of mixing wings including a first mixing wing located at a most upstream side and a second mixing wing located downstream of the first mixing wing, peroxide feeder for feeding peroxide at a position upstream of the first mixing wing, a first flow path throttling mechanism capable of changing an area of a flow path for the polypropylene at a side downstream of the first mixing wing and upstream of the second mixing wing to thereby change a filling ratio of a polypropylene mixed by the first mixing wing, and a second flow path throttling mechanism capable of changing the area of the flow path for the polypropylene at the downstream side of the second mixing wing to thereby change a filling ratio of the polypropylene mixed by the second mixing wing; and mixing of the polypropylene and peroxide having a half-life equal to or longer than 10 seconds at 200°C using this mixing extruder, wherein, in the mixing, the flow path for the polypropylene located downstream of the first mixing wing is throttled by the first flow path throttling mechanism to thereby increase the filling ratio of the polypropylene at the first mixing wing; and, when the temperature of the polypropylene mixed by the most downstream one of the mixing wings (21, 22) is below a predetermined temperature, the flow path for the polypropylene located downstream of the second mixing wing is throttled by the second flow path throttling mechanism to increase the filling ratio of the polypropylene at the second mixing wing, thereby bringing the polypropylene mixed by the mixing wing at the most downstream side to or above the predetermined temperature.