Abstract: A shutter for large pressure vessels comprises a lid assembly (Y) constituted of a seal plate (1) formed of a flexible material of a size fit for the opening of a pressure vessel and a multitude of beam members (2) attached to the outer surface of the plate (1) and extending widthwise. The beam members are connected to the seal plate in such a manner as to be parallel to each other along a reference line for connection, extending in the widthwise direction of the seal plate, and advance or retreat while flexing along a guide member (7) having at least one bending portion (71). The guide member extends between a closing position (A) disposed in front of the opening portion and a storage position (B) disposed on an outer housing of the pressure vessel, so that the assembly can be housed in a relatively small space. This shutter is most suitable for use on a medical autoclave and a retort for food processing.