An optical element having an optical multi-layer coating which can function as a phase plate without using quartz crystal or an optical film. The optical multi-layer coating is deposited on a plane which is set up on the optical element by way of a perpendicularly intersecting plane located on perpendicularly intersecting X- and Y-axes in perpendicularly intersecting relation with optical axis of an incident light beam on the optical multi-layer coating, the perpendicularly intersecting plane being inclined through a predetermined angle about the X- and/or Y-axis to establish a phase differential between two linearly polarized components of the incident light beam.