Torsion-Bar Antenna (TOBA) is a ground-based gravitational-wave detector for low frequency.
TOBA can detect intermediate-mass black hole binary mergers, gravitational wave stochastic background, and Newtonian noise.
TOBA is also useful for earthquake early warning.
TOBA consists of two 10-m test masses suspended horizontally and we aim to detect the torsional rotation of pendulums caused by tidal forces.
The resonant frequency of torsional motion is $\sim 1~\mathrm{mHz}$,
therefore TOBA has good design sensitivity of $1\times10^{-19}~\mathrm{/\sqrt{Hz}}$ between $0.1~\mathrm{Hz}$ and $10~\mathrm{Hz}$.
We are currently developing the third prototype detector Phase-III TOBA with 35 cm-scale pendulums at cryogenic temperature to demonstrate noise reduction.
The target sensitivity is set to $1\times10^{-15}~\mathrm{/\sqrt{Hz}}$.
In this paper, we will present the principle and science of TOBA and the status of the development of Phase-III TOBA.