PURPOSE: To efficiently preheat molten metal, such as aluminum. flowing in a groove to a high temp. to the extent of not solidifying the molten metal by disposing a cap disposed with a heating element above the trough.

CONSTITUTION: This trough for molten metal has the trough 10 consisting of a heat resistant member formed with the groove 12 for passing the molten metal and the cap 20 of a heat resistant member to be put atop the trough consisting of the heat resistant member. The melt of the metal is made to flow in the groove 12 of the trough and the heating element 30 is arranged on the inside surface side of the cap 20 consisting of the heat resistant member to heat the molten metal flowing in the groove. The inner side surface of this cap 20 consisting of the heat resistant member is formed to an arch shape curved upward at its sectional center and the heating element 30 is disposed on this curved surface, by which the area to mount the heating element 30 is increased and the heat quantity for heating is increased. The molten metal flowing in the groove is thus preheated to the high temp. to the extent of not solidifying the molten metal without increasing the temp. of the heating element 30 so much.