  It is generally believed that Type Ia supernovae are thermonuclear explosions
of carbon-oxygen white dwarfs (WDs). However, there is currently no consensus
regarding the events leading to the explosion. A binary WD (WD-WD) merger is a
possible progenitor of Type Ia supernovae. Space-based gravitational wave (GW)
detectors with considerable sensitivity in the deci-Hz range such as the
DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) can observe
WD-WD mergers directly. Therefore, access to the deci-Hz band of GWs would
enable multi-messenger observations of Type Ia supernovae to determine their
progenitor and explosion mechanism. In this paper, we consider the event rate
of WD-WD mergers and minimum detection range to observe one WD-WD merger per
year, using a nearby galaxy catalog and the relation between the Ia supernova
and host galaxy. Furthermore, we calculate DECIGO's ability to localize WD-WD
mergers and to determine the masses of binary mergers. We estimate that the
deci-Hz GW observatory can detect GWs with amplitudes $h\sim10^{-20}[\rm
Hz^{-1/2}]$ at 0.01-0.1 Hz, which is 1000 times higher than the detection limit
of DECIGO. Assuming progenitors of Ia supernovae are merging WD-WD
($1M_{\odot}-0.8M_{\odot}$), DECIGO is expected to detect 6600 WD-WD mergers
within $z=0.08$, and identify the host galaxy of such WD-WD mergers within
$z\sim0.065$ using GW detection alone.
