The invention provides a pressure adjustment device in a warmer, and a cover for the pressure adjustment device. Even if the warmer is used under the condition of not according with a right hot water quantity, the warmer is not liable to deform and can use safely for a long time. The warmer, composed of a warmer main body and the cover disposed on a water injection mouth of the warmer main body, is arranged in a manner that a proper part of the warmer is at least provided with one through hole (19) passing through the interior and exterior of the warmer, so as to prevent hot water from becoming cold because of the use of the warmer and prevent the warmer main body (10) from deforming because of internal pressure reduction. A ventilating mechanism (21) made of raw materials, which allow steam to pass and do not allow hot water to pass, is used for covering the through hole (19).
