A hydrogen generator comprising: a reformer which generates a hydrogen-containing gas from a source gas and reforming water; a condensed water channel through which condensed water generated by cooling of a steam-containing gas flows, wherein the condensed water contain a component that contaminates a reforming catalyst; a circulating water channel through which circulating water flows; an ion exchange resin filter which is provided to the circulating water channel such that the circulating water flowing in the circulating water channel is deionized by the ion exchange resin filter; characterized by a single reservoir tank (30, 30A, 30B, 30C, 30D), inside of which is divided into a first reservoir provided to the condensed water channel and a second reservoir provided to the circulating water channel; a first communicator (37, 37A, 37B) through which the first and second reservoirs (31, 32) are in communication with each other; and a reforming water channel which extends from a junction of the circulating water channel and supplies the circulating water as reforming water to the reformer, wherein the first reservoir has a first inlet from which the condensed water flows into the first reservoir and a first outlet from which the condensed water overflows to the outside of the single reservoir tank (30, 30A, 30B, 30C, 30D), the second reservoir has a second inlet from which the circulating water flows into the second reservoir and a second outlet of which the circulating water flows out of the second reservoir, the condensed water channel is connected only to the first reservoir out of the first and second reservoirs (31, 32), the first communicator (37, 37A, 37B) is provided below the first outlet, the single reservoir tank (30, 30A, 30B, 30C, 30D) is configured such that the pressure in the inner space of the first reservoir is maintained to be the same as the pressure in the inner space of the second reservoir, and the single reservoir tank (30A, 30C, 30D) further includes a second communicator, provided above the first outlet, through which the first reservoir and the second reservoir are in communication with each other. a reformer which generates a hydrogen-containing gas from a source gas and reforming water; a condensed water channel through which condensed water generated by cooling of a steam-containing gas flows, wherein the condensed water contain a component that contaminates a reforming catalyst; a circulating water channel through which circulating water flows; an ion exchange resin filter which is provided to the circulating water channel such that the circulating water flowing in the circulating water channel is deionized by the ion exchange resin filter; characterized by a single reservoir tank (30, 30A, 30B, 30C, 30D), inside of which is divided into a first reservoir provided to the condensed water channel and a second reservoir provided to the circulating water channel; a first communicator (37, 37A, 37B) through which the first and second reservoirs (31, 32) are in communication with each other; and a reforming water channel which extends from a junction of the circulating water channel and supplies the circulating water as reforming water to the reformer, wherein the first reservoir has a first inlet from which the condensed water flows into the first reservoir and a first outlet from which the condensed water overflows to the outside of the single reservoir tank (30, 30A, 30B, 30C, 30D), the second reservoir has a second inlet from which the circulating water flows into the second reservoir and a second outlet of which the circulating water flows out of the second reservoir, the condensed water channel is connected only to the first reservoir out of the first and second reservoirs (31, 32), the first communicator (37, 37A, 37B) is provided below the first outlet, the single reservoir tank (30, 30A, 30B, 30C, 30D) is configured such that the pressure in the inner space of the first reservoir is maintained to be the same as the pressure in the inner space of the second reservoir, and the single reservoir tank (30A, 30C, 30D) further includes a second communicator, provided above the first outlet, through which the first reservoir and the second reservoir are in communication with each other.