A milk basic protein fraction is heated if necessary, the resulting precipitate is removed to recover the resulting supernatant, alcohol is added to this supernatant, a fraction containing a high concentration of milk basic cystatin is recovered by removing the resulting precipitate and brought into contact with a carrier containing a sulfuric acid group, such as heparin, to recover a fraction which is not adsorbed onto the carrier, and this fraction is then treated with an ultrafiltration membrane having an cut-off molecular weight of 10-50 kDa to recover the permeate to produce a fraction containing a high concentration of milk basic cystatin. Alternatively, a milk-derived basic protein composition is brought into contact with an anion exchange resin to recover a fraction which is not adsorbed onto the resin, after which this fraction is brought into contact with a cation exchange resin to recover the target fraction by eluting a fraction which is adsorbed onto the resin with an eluent to produce a fraction containing a high concentration of milk basic cystatin. Further, a decomposition product of the fraction containing a high concentration of milk basic cystatin is produced by decomposing this fraction containing a high concentration of milk basic cystatin with protease.
