  We study the ionized gas spectrum of star forming regions in the Holmberg II
galaxy using the optical long-slit spectroscopic observations made at the 6-m
telescope of the Special Astrophysical Observatory of the Russian Academy of
Sciences (SAO RAS). We estimate the oxygen, nitrogen, sulphur, neon, and argon
abundances in individual HII regions and find the average metallicity in the
galaxy to be Z=0.1 or 0.3 Zsun depending on the estimation method employed. We
use these observations combined with the results of our earlier studies of the
Irr galaxy IC 10 and BCD galaxy VII Zw 403 to compare the currently most
popular methods of gas metallicity estimation in order to select among them the
techniques that are most reliable for analysing Irr galaxies. To this end, we
use the "direct" Te method and six empirical and theoretical methods. The
results of our observations mostly confirm the conclusions of Lopez-Sanchez et
al. (2012) based on the analysis of systematic deviations of metallicity
estimates derived by applying different methods to "model" HII regions.
