[Problem] To provide an injection device that suppresses screw whirling and reduces friction between a screw and a cylinder. [Solution] An injection device having a cylinder for heating a mold material and a screw that is rotatably provided inside the cylinder, wherein: the screw has a rotating shaft, a helical flight projecting from the outer circumference of the rotating shaft, and a helical dividing flight dividing the groove formed by the flight, and conveys the mold material from the upstream side to the downstream side along the rotating shaft as a result of rotation of the rotating shaft; the flight has a variable-diameter flight section in which the crest height in a downstream-side end section is lower than the crest height in an upstream-side end section; and the upstream end of the dividing flight is positioned on the upstream side of the downstream end of the variable-diameter flight section.