  The observations of jet breaks in the afterglows of short gamma-ray bursts
(SGRBs) indicate that the jet has a small opening angle of < 10{\deg}. The
collimation mechanism of the jet is a longstanding theoretical problem. We
numerically analyze the jet propagation in the material ejected by double
neutron star merger, and demonstrate that if the ejecta mass is > 10^{-2}
M_{sun}, the jet is well confined by the cocoon and emerges from the ejecta
with the required collimation angle. Our results also suggest that there are
some populations of choked (failed) SGRBs or low-luminous new types of event.
By constructing a model for SGRB 130603B, which is associated with the first
kilonova/macronova can- didate, we infer that the equation-of-state of neutron
stars would be soft enough to provide sufficient ejecta to collimate the jet,
if this event was associated with a double neutron star merger.
