  Almost all black holes (BHs) and BH candidates in our Galaxy have been
discovered as soft X-ray transients, so-called X-ray novae. X-ray novae are
usually considered to arise from binary systems. Here we propose that X-ray
novae are also caused by isolated single BHs. We calculate the distribution of
the accretion rate from interstellar matter to isolated BHs, and find that BHs
in molecular clouds satisfy the condition of the hydrogen-ionizaiton disk
instability, which results in X-ray novae. The estimated event rate is
consistent with the observed one. Possible candidates include IGR J17454-2919,
XTE J1908-094, and SAX J1711.6-3808. Near infrared photometric and
spectroscopic follow-ups can exclude companion stars for a BH census in our
Galaxy.
