  The <V/Vmax> of the cosmological X-ray flashes detected by Wide Field Cameras
on BeppoSAX is calculted theoretically in a simple jet model. The total
emission energy from the jet is assumed to be constant. We find that if the jet
opening half-angle is smaller than 0.03 radian, the theoretical <V/Vmax> is
less than ~0.4, which is consistent with the recently reported observational
value of 0.27+/-0.16 at the 1-sigma level. This suggests that the off-axis GRB
jet with the small opening half-angle at the cosmological distance can be
identified as the cosmological X-ray flash.
