There is provided an instantaneous water heater capable of expelling heated water or steam when internal pressure reaches or exceeds a predetermined pressure, thus preventing an accident, product damage, or the like. The instantaneous water heater includes a main body having an inlet allowing water to be introduced therethrough and an outlet allowing water to be discharged therethrough; a heating unit disposed in the main body and heating water introduced to the main body; and a decompression unit connected to a decompression hole disposed in the main body and expelling heated water or steam to the outside thereof when internal pressure within the main body reaches or exceeds a predetermined pressure.