An optical encoder comprising: a scale having diffraction gratings formed at predetermined pitches in a measurement axis direction; a detection head relatively movable with respect to the scale, the detection head including a light source portion configured to irradiate the scale with light, and a plurality of receiver portions configured to receive light reflected by or transmitted through the diffraction gratings of the scale, at different phases wherein; the plurality of receiver portions are four receiver portions placed respectively in upper left, upper right, lower left, and lower right places centered at the light source portion, the four receiver portions being configured to receive light reflected by or transmitted through diffraction gratings of the scale that are different in spatial phase by 90[deg.] from each other; and a signal processing device including a three-phase signal producer configured to perform differential amplification to light reception signals output from the four receiver portions to generate three phase differential signals which have a phase difference of 90 degrees with respect to each other; the signal processing device further comprising a quadrature differential signal producer configured to perform vector synthesis of the three phase signals to generate quadrature differential signals having a phase difference of 90 degrees; wherein the signal processing device is configured to calculate posture monitor signals corresponding to a Lissajous radius of the quadrature differential signals or to a squaring value R 2 of the Lissajous radius in order to detect Yaw misalignment of the detection head with respect to the scale. a scale having diffraction gratings formed at predetermined pitches in a measurement axis direction; a detection head relatively movable with respect to the scale, the detection head including a light source portion configured to irradiate the scale with light, and a plurality of receiver portions configured to receive light reflected by or transmitted through the diffraction gratings of the scale, at different phases wherein; the plurality of receiver portions are four receiver portions placed respectively in upper left, upper right, lower left, and lower right places centered at the light source portion, the four receiver portions being configured to receive light reflected by or transmitted through diffraction gratings of the scale that are different in spatial phase by 90[deg.] from each other; and a signal processing device including a three-phase signal producer configured to perform differential amplification to light reception signals output from the four receiver portions to generate three phase differential signals which have a phase difference of 90 degrees with respect to each other; the signal processing device further comprising a quadrature differential signal producer configured to perform vector synthesis of the three phase signals to generate quadrature differential signals having a phase difference of 90 degrees; wherein the signal processing device is configured to calculate posture monitor signals corresponding to a Lissajous radius of the quadrature differential signals or to a squaring value R 2 of the Lissajous radius in order to detect Yaw misalignment of the detection head with respect to the scale.