The present invention provides a substrate of TGase represented by (fluorescent group)-(linker)-(a portion containing a Gln residue capable of recognition by transglutaminase (TGase))-R, wherein the fluorescent group is fluorescein isothiocyanate (FITC), Texas Red (TE) or dansyl (Dns) or a group derived therefrom; the linker is a group represented by —NH—(CH2)n—CO— (n is an integer of 1 to 6); the portion containing a Gln residue capable of recognition by TGase is a group derived from a peptide selected from among QG and the like; and R is a hydroxyl group, or biotin, nucleic acid, azide, alkyne, maleimide or cyclopentadiene, or a group derived therefrom.