A flex spline 1 used for a meshing type gear device includes an attachment
portion 1a, a thin-thickness tubular portion 1b continuing
to the attachment portion and capable of being elastically deformed, and external
teeth 1c formed at the tip end portion of the tubular portion away
from the attachment portion. The thin-thickness tubular portion 1b is
formed in a conical shape with an opening angle which is tapered in a direction
from the attachment portion 1a toward the external teeth 1c
before inserting a wave generator 3.
