In this paper, we review a system of D-brane and anti-D-brane in type II superstring theories. [A. Sen, hep-th/9904207 and references there in; Y. Hyakutake. Master-Th., Doctor-Th. (in Japanese)] This system is unstable and tachyonic modes, which have negative mass squared, appear from open strings between D-brane and anti-D-brane. The effective field theory on the world-volume is described by U(1)×U(1) gauge theory with a complex tachyon field. Since the mass squared of the tachyon field is negative, a tachyon potential would be like a wine bottle. In order to make the system stable, the tachyon rolls down the potential and gets some vacuum expectation value. This is called the tachyon condensation mechanism. During this mechanism. Dp-brane and antr-Dp-brane annihilate completely, if we admit Sen's conjecture. The suspicions between tachyon condensation and Hawking radiation are also discussed.