Specific and accurate immunoassay of tartrate resistant acid phosphatase 5b (TRACP 5b) in a specimen may be carried out by bonding TRACP 5b in the specimen to an antibody, subjecting the TRACP 5b bonded to the antibody to enzyme reaction with a 2-halo-4-nitrophenylphosphoric acid or a salt thereof, a substrate for TRACP 5b, and then assaying the enzymatic activity of TRACP 5b.