A heating device comprising: a housing having an inlet port for allowing fluid to flow in and an outlet port for allowing the fluid to flow out; a heat exchange flow path placed inside the housing, the heat exchange flow path communicating with the inlet port; a heater for heating the fluid; a temperature buffer part placed inside the housing, the temperature buffer part communicating with the outlet port; a plurality of through holes formed between the heat exchange flow path and the temperature buffer part, and communicating between the heat exchange flow path and the temperature buffer part; and a plurality of mixing ribs disposed in the temperature buffer part on a downstream side of the through holes to obstruct a water flow toward the outlet port, wherein the heater is placed in a position that enables the heater to heat the fluid inside the heat exchange flow path, the heat exchange flow path guides the fluid flowing in through the inlet port and allows the fluid after being heated to flow into the temperature buffer part through the through holes, the temperature buffer part guides the fluid flowing in through the through holes to the outlet port and characterized in that the number of the mixing ribs is equal to the number of the through holes, each of the plurality of mixing ribs has a substantially crescent cross-section, wherein the upstream side of each of the plurality of mixing ribs has a rounded projecting shape with no angular part, and the downstream side of each of the plurality of mixing ribs is formed in a substantially recessed shape toward the outlet port such that washing water flows along the rounded projecting shape of the mixing rib and then flows toward an inner side of the recessed part so as to be mixed. a housing having an inlet port for allowing fluid to flow in and an outlet port for allowing the fluid to flow out; a heat exchange flow path placed inside the housing, the heat exchange flow path communicating with the inlet port; a heater for heating the fluid; a temperature buffer part placed inside the housing, the temperature buffer part communicating with the outlet port; a plurality of through holes formed between the heat exchange flow path and the temperature buffer part, and communicating between the heat exchange flow path and the temperature buffer part; and a plurality of mixing ribs disposed in the temperature buffer part on a downstream side of the through holes to obstruct a water flow toward the outlet port, wherein the heater is placed in a position that enables the heater to heat the fluid inside the heat exchange flow path, the heat exchange flow path guides the fluid flowing in through the inlet port and allows the fluid after being heated to flow into the temperature buffer part through the through holes, the temperature buffer part guides the fluid flowing in through the through holes to the outlet port and characterized in that the number of the mixing ribs is equal to the number of the through holes, each of the plurality of mixing ribs has a substantially crescent cross-section, wherein the upstream side of each of the plurality of mixing ribs has a rounded projecting shape with no angular part, and the downstream side of each of the plurality of mixing ribs is formed in a substantially recessed shape toward the outlet port such that washing water flows along the rounded projecting shape of the mixing rib and then flows toward an inner side of the recessed part so as to be mixed.