The behavior of the excitation caused by longitudinal vortices shedding from two cruciform circular cylinders with an equal diameter d is investigated using a wind tunnel. When the gap s between two cylinders is less than d/2, longitudinal vortices shed periodically and a fluctuating lift force is exerted on the upstream cylinder. The nondimensional frequency, StM, of the longitudinal vortices is independent of the Reynolds number. The value of StM is 0.06<StM<0.09 when s/d<0.25 (region A) and StM≒0.04 when 0.25<s/d<0.5 (region B). The fluctuating lift force on the upstream cylinder is estimated from the pressure on the cylinder surface using a phase-averaging technique. The result shows that the flow near the crossing of the cylinder is dominated by four trailing vortices in the region A and by two necklace vortices in the region B. These vortices shed an anti-phase on the opposite side of the upstream cylinder, causing an alternative lift force to be exerted on it. The abrupt change of StM at s/d=0.25 is caused by the fact that these two types of vortices interchange their role to excite the upstream cylinder.