An injection screw comprise a rear end to a front end of an effective length of the screw compartmented in the order of a feed zone, a compression zone and a metering zone, wherein the metering zone is constituted by the first metering zone, the second metering zone and the relaxation zone. A root diameter of the second metering zone is formed in a smaller diameter than a root diameter of the first metering zone and a screw channel depth is set deeper than a screw channel depth of the first metering zone. Variations in a resin temperature at injecting is solved and melting and kneading through a entire screw further are improved by only partial changing of the configuration of the metering zone.
