This paper presents a small-signal theory of a two-dimensional domain initiation in a bulk effect device, and discussions are focused on a transverse extension of the initial domain. It is introduced that the transverse extension velocity is not so fast as compared with the longitudinal propagation velocity in a stage of the domain initiation. This is because the longitudinal propagation velocity is much faster than the stable domain propagation velocity, although the transverse extension velocity is also faster than the mature domain velocity. It is necessary to estimate a marginal distance for an immature domain to travel in a domain growth time when a logic element utilizing the transverse extension of the high field domain is designed.