PURPOSE: To provide a mold for casting a billet at high speed which can drastically increase the casting speed and has a long service life and capable of stable operation.

CONSTITUTION: This mold 10 has projecting bulge 12 outside at the upper part which is gradually reduced until it becomes almost usual square shape at the lower end in the shape of each inner shape. In the mold 10 casting the billet at a high speed oscillating in the vertical direction, pouring molten metal 19 from the upper part and executing the operation while pouring a little quantity of lubricating oil, one or more of horizontal grooves 16 or the air gap part composed of many dimples are arranged on the four surfaces of inner periphery within 400mm remaining the part 20mm apart from the corner part at the lower position form the lowermost position M of a meniscus in the stationary operational condition of the mold.