A package comprising: a tray having an opening and a top face extended around the opening; and a sheet sealed to the top face to hermetically close the opening, wherein: the sheet is sealed to the top face to form a seal portion that has a common portion extended along an outer shape of the tray and at least one steam passage portion externally releasing steam from inside the tray; the steam passage portion has an inner edge protruded from an inner edge of the common portion toward the opening so as to have an angled shape with a pointed tip, the inner edge of the steam passage portion having a portion configuring one side relative to the tip and a portion configuring another side relative to the tip, the both portions configuring the sides being curved and extended nearing each other toward the tip; the steam passage portion has an outer edge in a curved shape with a pointed tip, the shape being similar to that of the inner edge of the steam passage portion; and the steam passage portion includes a merging portion corresponding to a peak of the angled shape and located between the tip of the outer edge and the tip of the inner edge, and a non-merging portion located between the merging portion and the common portion, characterised in that the non-merging portion has a width narrowed from the common portion toward the merging portion, wherein the tray has a square shape and each side of the square comprises one of said steam passage portion having said pointed tip so that the force for peeling the seal portion is likely to be concentrated near the center of each side of the square tray . a tray having an opening and a top face extended around the opening; and a sheet sealed to the top face to hermetically close the opening, the sheet is sealed to the top face to form a seal portion that has a common portion extended along an outer shape of the tray and at least one steam passage portion externally releasing steam from inside the tray; the steam passage portion has an inner edge protruded from an inner edge of the common portion toward the opening so as to have an angled shape with a pointed tip, the inner edge of the steam passage portion having a portion configuring one side relative to the tip and a portion configuring another side relative to the tip, the both portions configuring the sides being curved and extended nearing each other toward the tip; the steam passage portion has an outer edge in a curved shape with a pointed tip, the shape being similar to that of the inner edge of the steam passage portion; and the steam passage portion includes a merging portion corresponding to a peak of the angled shape and located between the tip of the outer edge and the tip of the inner edge, and a non-merging portion located between the merging portion and the common portion, characterised in that the non-merging portion has a width narrowed from the common portion toward the merging portion, wherein the tray has a square shape and each side of the square comprises one of said steam passage portion having said pointed tip so that the force for peeling the seal portion is likely to be concentrated near the center of each side of the square tray . <EOS>