A sheet discriminator, which can be included in an image forming apparatus, includes a sheet loader on which a recording medium is loaded, an information detector including a light emitter to emit light to a surface of the recording medium loaded on the sheet loader and a light receiver to receive the light emitted by the light emitter and detecting information of the recording medium, a sheet distinguisher to distinguish a type of the recording medium based on the information detected by the information detector, and a detector body encasing at least the information detector therein and having an opening formed on an opposing face facing the sheet loader. The opposing face has an opening formed thereon. The opening has at least one arc-shaped side.