PURPOSE: To permit a through hole to be formed to ensure the passage of air and remove condensed water and simplify the formation of a tray by a method wherein a funnel-shaped hole is formed at a suitable place on the bottom surface of a recess pat and elastic walls are formed projecting downwards along the outer hole edge.

CONSTITUTION: A recess part 2 is provided at a suitable place of its bottom surface with an arbitrary number of funnel-shaped holes 10 and each hole 10 is provided with an annularly-shaped elastic wall 11 formed continuously around its outer edge. By this method, the diameter of a suction hole 24 provided on the bottom surface of a cavity 23 of an internal die 21 is enlarged to the maximum possible extent. Due to the degree of vacuum increased to give a shape to a tray 1 and owing to the vacuum built up by the suction of air through a suction hole 24, the bottom surface corresponding to the recess part 2 is torn off instantaneously. In this way an arbitrary number of holes 10 can be easily formed in the bottom of the recess part 2. Therefore, the hole 10 formed under vacuum assumes a funnel-shaped torn edge projecting downwards along the outer edge.