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.提供被期待有用於癌之診斷，特別是大腸癌、乳癌、肺癌、甲狀腺癌或此等經轉移之癌的診斷的抗人類CEACAM5抗體Fab片段，及使用包含該Fab片段之複合體的診斷手段。 含有包含序列編號2之胺基酸編號1至121所示之胺基酸序列所構成之重鏈可變區域的重鏈片段，及包含序列編號4之胺基酸編號1至112所示之胺基酸序列所構成之輕鏈可變區域的輕鏈之抗人類CEACAM5抗體Fab片段，及包含該Fab片段之複合體。
