To provide a double blank detection device capable of detecting any double blank caused when conveying sheets and measuring a sheet thickness without stopping the movement of the sheet.

The double blank detection device comprises a CCD camera 1 for photographing the image of reflected light from the sheet 10, a stroboscopic device 2 for reflecting the reflected light by irradiating the sheet 10 with stroboscopic light, a power source 3 for stroboscopic light for sending out the stroboscopic light of the stroboscopic device 2, an image processor 4 for calculating the thickness of the sheet 10 from the processed result by acquiring the imaged signal of the reflected light photographed by the CCD camera 1, a sheet passage detection sensor 5 for detecting that the sheet 10 reaches the position of measurement, and a PLC (Programmable Logic Array) 6 for controlling the operation of the entire component and outputting the thickness abnormality signal Sda.