The purpose of the present invention is to provide: an anti-human CEACAM5 antibody Fab fragment which is expected as useful in diagnosing cancer, in particular, colon cancer, breast cancer, lung cancer, thyroid cancer or cancer metastasized therefrom； and a diagnostic means wherein a conjugate comprising the Fab fragment is used. An anti-human CEACAM5 antibody Fab fragment which comprises a heavy chain fragment containing a heavy chain variable region having an amino acid sequence of amino acid Nos. 1-121 in SEQ ID NO: 2, and a light chain containing a light chain variable region having an amino acid sequence of amino acid Nos. 1-112 in SEQ ID NO: 4； and a conjugate comprising the Fab fragment.
