A polarization direction measuring apparatus includes: a first polarizing plate having an unknown polarization direction about a reference axis; a sample whose polarization direction is to be measured; a rotatable sample holder on which the sample is mounted in a first direction and a second direction opposite to the first direction, wherein the sample holder rotates the sample along a reference axis in the azimuth direction; a light source that generates light passing though the first polarizing plate and the sample; and a light detector detecting light generated by the light source that passes though the first polarizing plate and the sample.