PURPOSE: To produce a cast ingot of a uniform shape, casting surface, ingot structure, etc. in horizontal continuous casting by solidifying the unsolidified molten metal in the cast ingot while agitating the same by acting rotating magnetic field upon the ingot.

CONSTITUTION: The molten metal 3 of Al or Al alloy in a tundish 2 is supplied through the opening part 4a of a baffle plate 4 into a mold 1 having a cooling water chamber la, where it is cooled to solidify and from which it is removed as a cast ingot 5 by means of tables 6 and pinch rolls 7. A rotary magnetic field device 11 is disposed around the ingot 5 in proximity to the outlet of said mold, whereby rotating magnetic fields are generated. While unsolidified metal 8 is agitated by the rotating magnetic fields, it solidifies. Therefore, the temp. distributions in the circumferential direction are made uniform, and a solidification end point 10 is maintained at the center of the cross section of the ingot 5, whereby the ingot of a uniform shape is obtained and the uniform and fine ingot structure is obtained.