Almost all black holes (BHs) and BH candidates in our Galaxy have been discovered assoft X-ray transients, so-called X-ray novae. X-ray novae are usually considered to arisefrom binary systems. Here, we propose that X-ray novae are also caused by isolated singleBHs. We calculate the distribution of the accretion rate from interstellar matter to isolatedBHs, and find that BHs in molecular clouds satisfy the condition of the hydrogen-ionizationdisc instability, which results in X-ray novae. The estimated event rate is consistent with theobserved one. We also check an X-ray novae catalogue (Corral-Santana et al.) and find that16/59 ~ 0.27 of the observed X-ray novae are potentially powered by isolated BHs. Thepossible 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 aBH census in our Galaxy.