PURPOSE: To prevent ice from protruding without reducing transparency of ice by so connecting the outer faces of adjacent small partitions of the outer periphery of an upper tray with ribs as to form an air layer in the partition wall of the rear face of the upper tray, and forming the ribs obliquely with respect to the outer peripheral flange of the upper tray.

CONSTITUTION: When an upper tray 2 is superposed on a lower tray 2 and the outer peripheral flange 14 of the tray 2 is pressed, an engaging pawl 16 is engaged with engaging holes 18, 19 so that the tray 2 is not, even if water is poured, floated. After the water is poured, the small partitions 5 of the tray 2 are filled with the water, the water is then fed into a lower tray 3 via small holes 7 so that both the trays 2 and 3 are filled with the water. However, since ribs 12 are provided on the rear face of the partition walls 4, the water is not fed to form an air layer. An ice making vessel 1 is placed on a heat insulating plate 21 set in a freezing chamber of a refrigerator in this state. When cooling is started, the water starts freezing from the water surface and the inner side faces 11 of the partitions 5 to seal the water therein. In this case, ice generated at an upper layer is intruded into a protrusion 15 formed on the inner side face 11 of the partition 5 to be sufficiently durable against the rise of hydraulic pressure due to the volumetric expansion at the time of icing and not raised upward.