A securing surface of a housing comprises a deformation region, which deforms radially outward by a predetermined quantity during press fitting of a thrust member, and another region. The deformation region has a tapered shape that gradually reduces in diameter in the downward direction, and is receded radially inward relative to the other region of the securing surface, by an amount equivalent to the amount of outward deformation thereof during press fitting. When the thrust member is press fitted into the inner periphery of a press fitting portion of the housing, the securing surface of the housing becomes substantially straight along entire axial region L thereof.