Olivine is a major mineral in ultramafic and mafic rocks. Due to the higher Mg/Fe partition coefficient of olivine than the coexisting phases, the occurrences of Mg-poorer olivine grains, especially with Mg#[= Mg/(Mg + Fe2+)] = 0.2-0.6, are rarely reported, and thus, their spectroscopic data are limited. Fe-rich olivine phenocrysts showing compositional zoning with Mg# = similar to 0.5 (core) and similar to 0.3 (rim) and microphenocrysts with Mg# = similar to 0.4 (core) and similar to 0.2 (rim) occur in basaltic trachyandesite of Miocene Shitara volcanic rocks in central Japan. These olivine grains were investigated by Raman spectroscopy. Combining our data with the published values, we have revised the equation for Mg#-Raman spectrum relationship proposed by Mouri and Enami (2008) as follows: Mg# = 0.005446 omega(2) - 0.20259 omega + 1.8442 (correlation coefficient r(2) = 0.984), where omega is the difference between the doublet peak positions (kappa(2) - kappa(1)).