holes 66 being a small diameter are formed in window 62 a part of the second holes are covered by edge portion 70 and the second holes are positioned only in the outermost rim of the window as a result second holes 66 do not substantially disturb the user to see food through window 62 since window 62 is formed incorporated with flange 72 a sufficient reinforcement effort of door 14 can be obtained manufacture can be achieved at low cost when compared with types with separate members although first hole 64 and second hole 66 have different diameter since the first hole and the second hole are punched in a single process an additional process is not required according to the inventor 39 s experiments it was proved that in a condition of a pitch p being 1 6 mm and a diameter of second hole 66 being 1 2 mm a diameter of first hole 64 can be 1 5 mm in the above condition an opening ratio is about 80 moreover with reference to fig1 a third hole 74 having a intermediate diameter between first hole 64 and second hole 66 may be provided between the first hole and the second hole for example a diameter of first hole 64 second hole 66 and third hole 74 is 1 4 mm 1 3 mm and 1 2 mm respectively more than three kinds of hole are available second hole 66 may be provided only four corners 73 of window 62 second hole 66 may be provided around window 62 as shown in fig1 in that case second hole 66 with hatching are punched in a difference process although only a few embodiments have been described in detail above those having ordinary skill in the art will certainly.