A photo mask for a lithography process enables the formation of high contrast image and facilitates manufacturing. The photo mask includes a transparent portion for an illumination light and a half-transparent part having a predetermined transmittance for the illumination light. These transparent and half-transparent portions are so constructed that a phase of a transmitted light beam from the half-transparent portion has a phase difference of 2 n.pi. or (2 n+1).pi. radian relative to a light beam passed through a transparent portion. Another type of photo mask includes transparent portion for the illumination light and a light shielding portion against the illumination light. At least a part of the shielding portion has a multilayer construction. An edge at least portion of the shielding portion has a different transmittance for the illumination light then a central portion surrounded by the edge portion.