Method of inspecting exposure system and exposure system

A method of inspecting an exposure system, the exposure system using a mask pattern comprising a first mask pattern and a second mask pattern, the first mask pattern being formed in a stripe having a line-and-space of a first pitch, the second mask pattern being disposed in parallel with the first mask pattern and formed in a stripe having a line-and-space of a second pitch different from the first pitch, the exposure system comprising a projection optical system for projecting illumination light to a substrate from a light source,

• the method comprising:
• illuminating the mask pattern with inspection light at a first angle with the optical axis of the illumination light, allowing the first mask pattern to diffract the inspection light to generate first diffraction light, and allowing the second mask pattern to diffract the inspection light to generate second diffraction light;
• measuring the relative distance between a first image due to the first mask pattern and a second image due to the second mask pattern, the first and second images being projected on the substrate via the projection optical system;
• and
• inspecting the condition of the projection optical system based on the relative distance,
• the first angle being set to allow the first diffraction light to be diffracted asymmetrically with respect to the optical axis into the projection optical system and the second diffraction light to be diffracted symmetrically with respect to the optical axis into the projection optical system.