Hierarchical structure formation scenario inevitably leads to the formation
of binary supermassive black holes (SMBHs) on a subparsec scale in
merged galactic nuclei. However, to date there has been no unambiguous
detection of such systems. In this talk, I propose how binary SMBHs
merge and a feasible method to detect binary SMBHs with a triple disk,
which consists of an accretion disk around each black hole and a
circumbinary disk surrounding them. The talk is divided into three parts:
First, the evolution of binary SMBHs is described based on the triple
disk model. Next, the number of binary SMBHs in nearby active galactic
nuclei is discussed. Finally, characteristic signals from such a binary
black-hole system are studied by Smoothed Particle Hydrodynamics
simulations.The detectability with next generation all-sky X-ray surveys
such as extended Roentgen Survey with an Imaging Telescope Array is
also briefly discussed.