An exothermic warmer comprising: an exothermic warmer main body that generates heat through an oxidation reaction and in which a flat-shaped exothermic element containing an oxidizable metal, an electrolyte, a carbon component, and water is covered with a first cover sheet and a second cover sheet, wherein the first cover sheet is a sheet that has an air permeability based on the Gurley measurement method JIS P8117 of 0 sec / (100 ml•6.42 cm 2) or more and 1500 sec / (100 ml•6.42 cm 2) or less and does not substantially limit the oxidation reaction, a water-retaining material is provided so as to be at least partially in contact with the exothermic element, and the exothermic warmer satisfies conditions to given below: a difference between an internal temperature of the exothermic element and a highest surface temperature of the exothermic warmer main body is 10°C or less; a highest skin temperature obtained when the exothermic warmer is applied to human skin is 38°C or more and 42°C or less; and a ratio of an amount (mg/cm 2 •10 min) of steam generated in 10 minutes after the oxidation reaction has started to a mass (g/cm 2) of the exothermic element takes a value of 50 or more and 250 or less, wherein, in a state before the oxidation reaction has started, the exothermic element has a water content of 9 mass% or more and 25 mass% or less, wherein the water-retaining material is disposed on a surface of the exothermic element that is on the first cover sheet side or a surface of the exothermic element that is on the second cover sheet side, wherein an outer jacket is provided on an outer side of the first cover sheet and/or the second cover sheet of the exothermic warmer main body, wherein a thickness of the exothermic warmer main body excluding the outer jacket in the exothermic warmer is 10% or more and 80% or less of a thickness of the exothermic warmer. an exothermic warmer main body that generates heat through an oxidation reaction and in which a flat-shaped exothermic element containing an oxidizable metal, an electrolyte, a carbon component, and water is covered with a first cover sheet and a second cover sheet, wherein the first cover sheet is a sheet that has an air permeability based on the Gurley measurement method JIS P8117 of 0 sec / (100 ml•6.42 cm 2) or more and 1500 sec / (100 ml•6.42 cm 2) or less and does not substantially limit the oxidation reaction, a water-retaining material is provided so as to be at least partially in contact with the exothermic element, and the exothermic warmer satisfies conditions to given below: a difference between an internal temperature of the exothermic element and a highest surface temperature of the exothermic warmer main body is 10°C or less; a highest skin temperature obtained when the exothermic warmer is applied to human skin is 38°C or more and 42°C or less; and a ratio of an amount (mg/cm 2 •10 min) of steam generated in 10 minutes after the oxidation reaction has started to a mass (g/cm 2) of the exothermic element takes a value of 50 or more and 250 or less, a difference between an internal temperature of the exothermic element and a highest surface temperature of the exothermic warmer main body is 10°C or less; a highest skin temperature obtained when the exothermic warmer is applied to human skin is 38°C or more and 42°C or less; and a ratio of an amount (mg/cm 2 •10 min) of steam generated in 10 minutes after the oxidation reaction has started to a mass (g/cm 2) of the exothermic element takes a value of 50 or more and 250 or less, wherein, in a state before the oxidation reaction has started, the exothermic element has a water content of 9 mass% or more and 25 mass% or less, wherein the water-retaining material is disposed on a surface of the exothermic element that is on the first cover sheet side or a surface of the exothermic element that is on the second cover sheet side, wherein an outer jacket is provided on an outer side of the first cover sheet and/or the second cover sheet of the exothermic warmer main body, wherein a thickness of the exothermic warmer main body excluding the outer jacket in the exothermic warmer is 10% or more and 80% or less of a thickness of the exothermic warmer.