Disclosed is a substance which can be used for the treatment and prevention of an allergic disease, particularly a substance which can be used for the treatment and prevention of an allergic disease, does not induce any anaphylactic reaction in a living body, can cover an epitope in a patient suffering from an allergic disease, and/or can be produced readily in a large quantity and at high purity. Specifically disclosed is a modified protein which is an antigen protein capable of inducing the production of an IgE antibody in a mammal and having at least one cysteine residue. In the protein, each of all of the cysteine residues excluding one cysteine residue in an amino acid sequence constituting the protein is substituted by an amino acid residue which does not undergo the modification with a polyethylene glycol (PEG), and the remaining cysteine residue is modified with a PEG having a sufficient length for inhibiting the antigenicity of a B-cell epitope.
