Method for forming magnesium oxide thin film and processed plate

A preparation method for a magnesium oxide thin film which is a method for depositing a magnesium oxide thin film on a substrate by a laser abrasion method using a sintered body or single crystal of magnesium oxide as a target, wherein a flat processed film made of magnesium oxide having a (111) plane as its front surface is prepared, using a substrate made of strontium titanate having a (111) plane as its principal surface or yttria-stabilized zirconia having a (111) plane as its principal surface, by directly depositing a film on the principal surface of the substrate and epitaxially growing the film, an amount of laser energy in the laser abrasion method is 50 mJ or more, and the processed film is a film having a film thickness not exceeding 50 nm, and is a film having a root mean square roughness Rq of a magnesium oxide (111) plane not exceeding 0.5 nm.