The present invention is to provide a tissue slice selection method and an embedded block-preparing cassette which are capable of suppressing occurrence of errors in inspection results. The present invention makes it possible to suppress the occurrence of errors in inspection results by including: an embedded block-preparing process of preparing an embedded block by embedding both a biological tissue fragment and a standard substance in the same embedding agent; an embedded tissue slice preparing step of slicing the embedded block to prepare a sheet-like embedded tissue slice having a tissue slice and standard substance slices appearing on a surface thereof; and a tissue slice selecting step of selecting the tissue slice to be inspected, based on a light signal from the standard substance slice.