The detected surface (patch section) of the sheet Ps for detection, the bulkhead plate 91 and the density calibration reference section 99 fall within the focus depth L' of the color density sensor 80. The opening section 74 of the ejection side conveyance path 70 is covered except at color density detection so that heat and vapor of oil and wax originated form the sheet P through the fixing process do not reach color density sensor side.