To provide an overheating protection system of a pump that has a thin pipe diameter and a short bypass pipe, and that prevents a temperature of liquid from increasing at low cost and effectively.

The overheating protection system 20 of the pump includes: the bypass pipe 21 which communicates with a discharge pipe 13 and a suction pipe 12 of the pump 14 and through which part of the liquid flows from the discharge pipe 13 toward the suction pipe 12 during at least a shutoff operation; and a pressure reduction mechanism 22 which has a sealed tank 23 located on a route of the bypass pipe 21 and a vacuum pump 24 regulating a pressure of a space 23b formed in the sealed tank 23 to a value lower than the atmospheric pressure.