A cavity-retaining tool (230) for surgery of this invention is insertable into body tissues to be ready for use and comprises: a cavity-retaining means (231) which retains a cavity for operation works in the body; and a soft cylinder member (233) which communicates with the cavity retained by the cavity-retaining means and interconnects the space outside with the cavity, and is used for surgery, by allowing an endoscope (240) and operation tools (245) to be inserted through the soft cylinder member into the cavity retained by the cavity-retaining means. <IMAGE>