An image forming apparatus includes a feeder, a conveyance section, an image forming section, a light emitter, a light receiver, a measuring section, and a permeation depth calculator. The feeder feeds a sheet. The conveyance section conveys the sheet. The image forming section forms an image on the sheet conveyed by the conveyance section using an ink. The light emitter emits near-infrared light to the sheet with the image thereon. The light receiver receives reflected light of the near-infrared light. The reflected light has been reflected by the sheet with the image thereon. The measuring section measures a received light quantity of the reflected light. The permeation depth calculator calculates a permeation depth of the ink permeating the sheet based on the received light quantity.