  Expanding the observational frequency of gravitational waves is important for
the future of astronomy. Torsion-Bar Antenna (TOBA) is a mid-frequency and
low-frequency gravitational wave detector using a torsion pendulum. The low
resonant frequency of the rotational mode of the torsion pendulum enables
ground-based observations. The overview of TOBA, including the past and present
status of the prototype development is summarized in this paper.
