  Ring structures are observed by (sub-)millimeter dust continuum emission in
various circumstellar disks from early stages of Class 0 and I to late stage of
Class II young stellar objects (YSOs). In this paper, we study one of the
possible scenarios of such ring formation in early stage, which is coagulation
of dust aggregates. The dust grains grow in an inside-out manner because the
growth timescale is roughly proportional to the orbital period. The boundary of
the dust evolution can be regarded as the growth front, where the growth time
is comparable to the disk age. With radiative transfer calculations based on
the dust coagulation model, we find that the growth front can be observed as a
ring structure because dust surface density is sharply changed at this
position. Furthermore, we confirm that the observed ring positions in the YSOs
with an age of $\lesssim1$ Myr are consistent with the growth front. The growth
front could be important to create the ring structure in particular for early
stage of the disk evolution such as Class 0 and I sources.
