clipped by the basic material 12 and the screen 24 and is capable of sliding there are opened windows 27a whose positions correspond to the through holes 20 of the slat 11 in the shutting plate 27 it is preferable to provide two reinforcing plates 27b at the mid section of each window 27a so as to reinforce the shutting plate 27 see fig9 the status in which the through holes 20 of the basic material 12 coincide with the windows 27a of the shutting plate 27 and in which ventilation can be executed is shown in fig1 the status is which the windows 27a are shifted from the through holes 20 to close the through holes 20 is shown in fig8 it is necessary that the length n of the through holes 20 or the windows 27a is longer than the distance m between the through holes 20 or between the windows 27a viz n gt m note that shutting plate 27 is slidably inserted between the screen 24 and the basic material 12 but the screen 24 may be inserted between the shutting plate 27 and the basic material 12 guides 22s whose structure is the same as the above noted guides 22 may be provided between the through holes 20 in case that the distance between the through holes 20 is long see fig1 in case of fig1 the screen 24 and the shutting plate 27 never slacken at the midway section thereof further similar to the example shown in fig7 the piled part 24a see fig7 of the screen 24.