A medium discrimination device includes a transmitted light quantity measurement device arranged to measure a transmitted light quantity in a thickness direction of a printing medium on a conveyance route, and configured to perform a measurement of the transmitted light quantity at each time the printing medium is conveyed, a measurement value memory device storing a result of the measurement performed by the transmitted light quantity measurement device, a medium setting device configured to previously set a type of printing medium to be used, a medium information memory device storing information of a transmitted light quantity assigned to each printing medium to be set by the medium setting device, and a control processing unit configured to perform paper type detection on a first printing medium, and double feed detection on following printing mediums.