To provide a vibration reduction system capable of suppressing an excessive increase in vibration and eliminating hunting when hunting starts.

A vibration reduction system includes: a support 70, of which one end side part is connected to a drain tank 63 of a heat medium heater 60 and the other end side part is connected to a structure 92 accommodating the drain tank 63 to support the drain tank 63 with respect to the structure 92, and which is expandable in a direction connecting the one end and the other end of the support; and a support expansion unit 80 connected to a control unit 91 and expanding and contracting the support 70. The control unit 91 controls the support expansion unit 80 so as to expand or contract the support 70 in the direction connecting the one end and the other end of the support 70, if a water level detected by a level meter 64 exceeds a predetermined range more than predetermined times within a predetermined time period.