To provide a water heater used as a mist sauna capable of providing comfortable mist sauna and supplying hot water even during the use of the mist sauna.

This water heater used as the mist sauna provided with a mist sauna pipe conduit 2 connected with a mist nozzle 20 of a bathroom BR to a heat source machine 1, further comprises a bypass pipe conduit 61 bypassed from a water supply pipe conduit 6 and connected with a tapping pipe conduit 7, a mixing valve 9 mounted on a junction part of the bypass pipe conduit 61 and the tapping pipe conduit 7, and mixing and adjusting the water from the bypass pipe conduit 61 and the hot water from the tapping pipe conduit 7 to adjust a temperature to a mist set temperature, an opening/closing valve 21 mounted on the mist sauna pipe conduit 2 for circulating or blocking the hot water of which the temperature is adjusted by the mixing valve 9, and a control portion 8 permitting the combustion of a burner when a water volume sensor S mounted on the water supply pipe conduit 6 detects specific working water volume, and permitting the combustion of the burner when the water volume sensor S detects small water volume not reaching the volume of working water when the mist sauna is used in a state of opening the opening/closing valve 21.