PURPOSE: To detect the presence of a medium even if the medium has a high light transmittance.

CONSTITUTION: The medium detecting device 1 is provided with a medium carrying path 35, medium carrying means 2, and optical sensor device 10 which detects the presence/absence of a medium 34 and is equipped with light emitting/ receiving elements 22 and 23 and prism 12. The carrying path 35 is formed out of a lower carrying guide 31 and upper carrying guide 33 counterposed to the guide 31. The prism 12 reflects the light emitted from the light emitting element 22 toward the light receiving element 23. The emitted and reflected light rays are extended in the directions intersecting the surface of the medium 34 and the front end 34a of the medium 34 in the transfer direction crosses the emitted and reflected light rays. Therefore, the presence of the medium 34 can be detected even when the medium 34 has high light transmittance, because the quantity of the light received by the element 23 is attenuated by the front end 34a of the medium 34.