  We investigate the relation between stellar mass ($M_\star$), star formation
rate (SFR), and metallicity ($Z$) of galaxies, so called the fundamental
metallicity relation, in the galaxy sample of the Sloan Digital Sky Survey Data
Release 7. We separate the galaxies into narrow redshift bins and compare the
relation at different redshifts, and find statistically significant ($> 99$%)
evolution. We test various observational effects that might cause seeming $Z$
evolution, and find it difficult to explain the evolution of the relation only
by the observational effects. In the current sample of low redshift galaxies,
galaxies with different $M_\star$ and SFR are sampled from different redshifts,
and there is degeneracy between $M_\star$/SFR and redshift. Hence it is not
straightforward to distinguish a relation between $Z$ and SFR from a relation
between $Z$ and redshift. The separation of the intrinsic relation from the
redshift evolution effect is a crucial issue to understand evolution of
galaxies.
