A cassette for fixing, embedding and slicing biological tissues includes a base sheet and a lid sheet formed of an elongated plastic sheet which is sliceable and has not less than predetermined strength. The base sheet has grooves as biological tissue receiving portions. The base plate and the lid sheet are formed by folding the plastic sheet along a fold line so that the lid sheet can be selectively opened and closed. Biological tissues are pressed against the bottoms of the grooves with pressing/retaining sponge members secured to the lid sheet at its portions corresponding to the respective grooves. In this state, the biological tissues are fixed by a fixing agent, and the cassette is dehydrated, infiltrated with paraffin as an embedding agent to embed the cassette, thereby forming a block including an embedding frame. The block is then sliced with a microtome to provide good biological tissue specimens.