each of fig5 a through 5c illustrates the light emitting device 50 and the light receiving device 60 that are bonded to the substrate 40 in fig5 a through 5c the light emitting device 50 includes four light emitters 51 and the light receiving device 60 includes four light receivers 61 in fig5 a through 5c each dashed dotted line connects the four light emitters 51 and the four light receivers 61 as illustrated by fig5 a the light emitting device 50 and the light receiving device 60 are ideally or preferably bonded to the substrate 40 such that each of the light emitters 51 is positioned in the center of the corresponding one of four through holes 41 a and each of the light receivers 61 is positioned in the center of the corresponding one of four through holes 41 b with this alignment the loss of light is small in fig5 a the light emitters 51 and the light receivers 61 are collinear in the above process however the step of aligning the through holes 41 a with the light emitters 51 and bonding the light emitting device 50 to the substrate 40 is performed separately from the step of aligning the through holes 41 b with the light receivers 61 and bonding the light receiving device 60.