A heat storage mechanism, of a heat exchanger, which does not shorten the service life of the heat exchanger even when a facility using the heat exchanger is intermittently operated and which suppresses a decrease in efficiency at the time of restart of the facility, is provided. The heat storage mechanism for storing heat of a heat exchanger during stop of operation of a facility provided with the heat exchanger includes an outflow prevention unit configured to prevent outflow of an exhaust gas to the outside, which is a heating medium of the heat exchanger, during stop of operation of the facility is provided in an exhaust passage through which the exhaust gas is discharged to the outside.