A plasticizing screw for injection molding that is configured to be installed in an injection molding machine which performs injection molding by plasticizing granular thermoplastic resin feedstock containing reinforcement fibers, the plasticizing screw for injection molding comprising: a shaft which is rotationally driven; a flight (12; 17) spirally provided on the peripheral surface of the shaft; and a mixer (13;18) which is provided further to the leading end side in an axial direction than the flight and has a plurality of fins (131a, 132a, 133a; 181a, 182a, 183a) radially protruding from the peripheral surface of the shaft and provided in a circumferential direction, wherein the mixer is provided in plural stages (131, 132, 133; 181, 182, 183) in the axial direction and the number of fins of the mixer of each stage increases toward the leading end side in the axial direction from a rear end side in the axial direction, the protrusion height (H1, H2, H3) of each fin from the peripheral surface of the shaft is lower than the protrusion height (Hf) of the flight (12; 17) from the peripheral surface of the shaft, the mixer (13; 18) is provided in three or more stages (131, 132, 133; 181, 182, 183) in the axial direction, and where the number of fins of the mixer of the first stage (133; 183) which is located on the rearmost end side in the axial direction is set to be N, the number of fins of the mixer of the second stage (132; 182) which is located next to the rear end side of the first stage in the axial direction is in a range of more than 1.5N and less than 2.5N, and the number of fins of the mixer of the third stage (131; 181) or later which is located further to the leading end side in the axial direction than the second stage is more than 3N. a shaft which is rotationally driven; a flight (12; 17) spirally provided on the peripheral surface of the shaft; and a mixer (13;18) which is provided further to the leading end side in an axial direction than the flight and has a plurality of fins (131a, 132a, 133a; 181a, 182a, 183a) radially protruding from the peripheral surface of the shaft and provided in a circumferential direction, wherein the mixer is provided in plural stages (131, 132, 133; 181, 182, 183) in the axial direction and the number of fins of the mixer of each stage increases toward the leading end side in the axial direction from a rear end side in the axial direction, the protrusion height (H1, H2, H3) of each fin from the peripheral surface of the shaft is lower than the protrusion height (Hf) of the flight (12; 17) from the peripheral surface of the shaft, the mixer (13; 18) is provided in three or more stages (131, 132, 133; 181, 182, 183) in the axial direction, and where the number of fins of the mixer of the first stage (133; 183) which is located on the rearmost end side in the axial direction is set to be N, the number of fins of the mixer of the second stage (132; 182) which is located next to the rear end side of the first stage in the axial direction is in a range of more than 1.5N and less than 2.5N, and the number of fins of the mixer of the third stage (131; 181) or later which is located further to the leading end side in the axial direction than the second stage is more than 3N.