The apparatus (3) has a temperature measuring device which measures the temperature actual value. A supply line delivers a tempering medium to the blow mold element and discharge line carries the tempering medium from the blow mold element. The blow mold elements are connected with a tempering circuit. An analyzing unit analyzes the received temperature actual value and received temperature target value. A calculating unit calculates the setting value for tempering device from received temperature actual value and the received temperature target value. An independent claim is included for method for controlling tempering device of blow-molding machine.