of fig7 is started transported document trailing edge interrupt processing in step 82 the second photosensor 30 is switched off and a times value t4 is determined in step 84 the timer value t3 stored at address r3 is subtracted from the timer value t4 and the result is written to a specific address r4 of the first memory section 38 next in step 86 the input document length of document 50a stored in the second memory section 40 at an address specified by pointer pread is retrieved by the retrieval section 42 of fig3 and compared by the comparator 44 of fig3 with the transported document length stored at address r4 if there is still a space between document 50a and document 50b such as there was when the documents were fed onto the transport member 26 of fig3 the document length as measured by the input document measurement means will be the same as the document length as measured by the transported document length measurement means in this case it is determined in the step 88 that document transport is proceeding normally and the process goes to step 92 in which the pointer pread is rewritten to specify the address from which the length of the following document 50b is to be retrieved for the comparison process it.