  The error region of the the gravitational-wave (GW) event GW151226 was
observed with Monitor of All-sky X-ray Image (MAXI). MAXI was operated at the
time of GW151226, and continuously observed to 4 minutes after the event. MAXI
covered about 84% of the 90 percent error region of the GW event during the
first 92 minutes orbit after the event. No significant X-ray transient was
detected in the GW error region. A typical 3-$\sigma$ GSC upper limit for a
scan is 1.2 $\times 10^{-9}$ ergs cm$^{-2}$ s$^{-1}$ in the 2-20 keV. The
auto-detection (MAXI nova-search) systems detected a short excess event with a
low significance (2.85$\sigma$) from 5257 s to 5260 s after the GW trigger.
Finally, we discuss the sensitivity of MAXI to long X-ray emissions of short
gamma-ray bursts, which are expected to accompany GW events.
