An object of the present invention is to provide a method for inhibiting activation of signaling pathway mediated by erbB1 or erbB2 in human cell and a signaling inhibitor to be used therefor. The above-described activation of signaling pathway can be inhibited by a polypeptide comprising at least one of PTB domain or ERK2 binding domain of human FRS2β. The above-described polypeptide may be introduced directly into cell, or nucleic acid which encodes for the above-described polypeptide may be introduced into cell to allow expression of the polypeptide in the cell. Such polypeptide and nucleic acid can be used, for example, as a signaling inhibitor. In addition, since erbB1 and erbB2 are involved in development of cancer, the above-described signaling inhibitor is also useful, for example, as an anticancer drug.