A refrigerant evaporator includes a first evaporation unit and a second evaporation unit disposed in series in a flow direction of fluid to be cooled by evaporating refrigerant. An intermediate tank portion through which refrigerant flows is connected to an outer surface of one tank portion of the first evaporation unit and an outer surface of one tank portion of the second evaporation unit. A tank external refrigerant space through which refrigerant flows is defined by an outer wall of the one tank portion of the first evaporation unit, an outer wall of the one tank portion of the second evaporation unit, and an outer wall of the intermediate tank portion.