A medium processing device is provided having a front panel which is formed with an aperture where a magnetic recording medium is inserted into or ejected from and which partitions an outside of a device from an inside of the device. A magnetic head is provided in the inside of the device for writing or reading information in or from the magnetic recording medium a shading filter is provided in at least one part of the front panel for shading visible light and a foreign matter detecting mechanism is provided on an inner side of the device with respect to the shading filter for detecting a foreign matter that is attached to the outside of the device.