  We study a membrane -- anti-membrane system in Matrix theory. It in fact
exhibits the tachyon instability. By suitably representing this configuration,
we obtain a (2+1)-dimensional U(2) gauge theory with a 't Hooft's twisted
boundary condition. We identify the tachyon field with a certain off-diagonal
element of the gauge fields in this model. Taking into account the boundary
conditions carefully, we can find vortex solutions which saturate the
Bogomol'nyi-type bound and manifest the tachyon condensation. We show that they
can be interpreted as gravitons in Matrix theory.
