Disclosed are: an inhibitor of the IgE-dependent activation of mast cells； a method for inhibiting the the IgE-dependent activation of mast cells； a pharmaceutical composition for treating and/or preventing allergic diseases； a method for treating and/or preventing allergic diseases； a phosphorylated peptide which can be used for the inhibitors and the methods； and others. The inhibitor of the IgE-dependent activation of mast cells is characterized by comprising a peptide (a) or (b), a derivative of the peptide, or a salt of the peptide or the derivative: (a) a peptide which comprises the amino acid sequence represented by formula (I): KVPEDRV-Y1-EELNI-Y2-SAT-Y3-SELEDPG； and (b) a peptide which is produced by deleting, substituting or adding one or several amino acid residues in the amino acid sequence represented by formula (I) and which has an inhibitory activity on the IgE-dependent activation of mast cells.
