Provided are an anti-human CEACAM5 antibody Fab fragment expected to be useful in the diagnosis of a cancer, particularly, the diagnosis of colorectal cancer, breast cancer, lung cancer, thyroid gland cancer or a cancer resulting from the metastasis thereof, and a diagnosis approach using a conjugate comprising the Fab fragment. The present invention provides an anti-human CEACAM5 antibody Fab fragment comprising a heavy chain fragment comprising a heavy chain variable region consisting of the amino acid sequence represented by amino acid positions 1 to 121 of SEQ ID NO: 2, and a light chain comprising a light chain variable region consisting of the amino acid sequence represented by amino acid positions 1 to 112 of SEQ ID NO: 4, and a conjugate comprising the Fab fragment.