Provided is a mutant protein capable of binding specifically and quickly to troponin I derived from human myocardium. The mutant protein comprises a first mutant scFv antibody fragment 51 a second mutant scFv antibody fragment 52 and a linker 53. The first mutant scFv antibody fragment 51 comprises a first light chain variable region 51L consisting of an amino acid sequence represented by SEQ ID NO: 76 and a first heavy chain variable region 51H consisting of an amino acid sequence represented by SEQ ID NO: 77. Similarly, the second mutant scFv antibody fragment 52 comprises amino acid sequences of SEQ ID NO: 78 and SEQ ID NO: 79. The linker 53 is provided between the C-terminus of the first heavy chain variable region 51H and the C-terminus of the second heavy chain variable region 52H.