A retainer comprises a roller retaining click which is bent from an inner diameter edge of a rib portion toward the inside in an axial direction and extends so as to protrude and positioned between adjacent rollers. The first coil group is classified into M phase sub coil groups, with the coils of the sub coil groups aligned one at a time in sequence at a specified sub coil group interval Dc from the first phase sub coil group to the M-th phase sub coil group. Forces may be attenuated to further optimize response.