Abstract: A system for controlling a temperature of a molten resin in a cylinder of an extruder includes a plurality of temperature detecting elements which are disposed in a wall of the cylinder generally in a plane perpendicular to a longitudinal axis of the cylinder to detect the temperature of the cylinder. The temperature detecting elements are radially spaced by different distances from an inner peripheral surface of the cylinder wall. Detecting devices are provided for detecting an ambient temperature of the extruder and the number of revolutions of a screw of the extruder. A microprocessor unit is programmed to be responsive to detecting signals from the temperature detecting elements and detecting devices to calculate a proper estimated temperature of the molten resin and to calculate a difference between a desired temperature of the resin and the estimated temperature and to determine in accordance with this difference a set temperature of the cylinder wall.