  A new detection method for ultra-low frequency gravitational waves (GWs) with
a frequency much lower than the observational range of pulsar timing arrays
(PTAs) was suggested in Yonemaru et al. (2016). In the PTA analysis, ultra-low
frequency GWs ($\lesssim 10^{-10}$ Hz) which evolve just linearly during the
observation time span are absorbed by the pulsar spin-down rates since both
have the same effect on the pulse arrival time. Therefore, such GWs cannot be
detected by the conventional method of PTAs. However, the bias on the observed
spin-down rates depends on relative direction of a pulsar and GW source and
shows a quadrupole pattern in the sky. Thus, if we divide the pulsars according
to the position in the sky and see the difference in the statistics of the
spin-down rates, ultra-low frequency GWs from a single source can be detected.
In this paper, we evaluate the potential of this method by Monte-Carlo
simulations and estimate the sensitivity, considering only the "Earth term"
while the "pulsar term" acts like random noise for GW frequencies
$10^{-13}-10^{-10}$ Hz. We find that with 3,000 milli-second pulsars, which are
expected to be discovered by a future survey with the Square Kilometre Array,
GWs with the derivative of amplitude of about $3 \times 10^{-19}~\rm{s}^{-1}$
can in principle be detected. Implications for possible supermassive binary
black holes in Sgr${}^*$ and M87 are also given.
