A glossiness detector includes a glossiness sensor, a glossiness corrector, and a reference plate. The glossiness corrector corrects glossiness detected by the glossiness sensor. The reference plate is used for correction processing of the glossiness sensor. Then, the glossiness corrector acquires a glossy reflection profile of the reference plate that has been detected in advance. The glossiness corrector corrects the glossiness according to the reference plate on the basis of the glossy reflection profile and a gloss distribution of the reference plate that has been detected by the glossiness sensor.