PURPOSE: To provide an injection molder, in which the disappearance of flexural deformation in a heating cylinder can be detected with a simple and low cost means and the starting of the rotation of a screw is made possible just after the point of time when the flexural deformation in the heating cylinder disappears.

CONSTITUTION: A first temperature detecting means 8 and a second temperature detecting means 9 are arranged in the positions, which are thermally unbalanced at the early heating period and nearly symmetrical about axis center line, at the tip side of a heating cylinder 2. After heating of the heating cylinder 2 with band heaters 4 under the above-mentioned state, when the measured value of the temperature with the first temperature detecting means 8 becomes nearly equal to that with the second temperature detecting means 9, a system controller 12 for performing the collective controlling of a machine judges the arrival of timing for starting the rotation of a screw 5.