A document photosensor is provided which comprises upper and lower sensor assemblies 1 and 2 disposed on opposite sides of a passageway 55 along which a document 50 is transported. Upper sensor assembly 1 comprises an upper substrate 11, an upper LED chip 21 surface-mounted on upper substrate 11 and an upper PD chip 37 surface-mounted on upper substrate 11. Lower sensor assembly 2 comprises a lower substrate 12, a lower LED chip 31 surface-mounted on lower substrate 12 and a lower PD chip 38 surface-mounted on lower substrate 12. These chips 21, 37, 31 and 38 are secured at precise locations on upper and lower substrates 11 and 12 with accuracy on the order of a few micrometers or less to exactly detect by upper and lower PD chips 37, 38 lights irradiated from upper and lower LED chips 21 and 31 after penetration of these lights through particular points on a bill 50 moved along passageway 55 to improve validation performance of bill 50.