To provide the subject new monitor for use in seizing the optimum washing time when a plate-type heat exchanger is to be washed by CIP operation such as in a sterilizer.

This monitor is intended for use in seizing the optimum washing time when a plate-type heat exchanger is to be washed by CIP operation such as in a sterilizer, being such one as to be equipped with thermometers such as thermocouples respectively mounted on stock liquid inlet side positions 2, 3, and outlet side positions 4, 5 and medium liquid inlet side positions 6, 7, and outlet side positions 8, 9 in the above-mentioned plate-type heat exchanger, and a personal computer having the function to perform file writing and graphic display through calculating thermal transmittance or scale coefficient based on temperature data received from the above thermometers and give an alarm on a display screen when the thermal transmittance or scale coefficient exceeds a given set level.