An object is to provide a frame number detecting device having a frame counter
which is less apt to output a wrong frame number even if a sync signal type was
erroneously detected from a DVD etc. and the combination of variation of the sync
signal types corresponds to an existing frame number. A sync number encoding portion
(6) specifies a read frame number and provides it as an output. First and
second frame counters (7) and (13) run by themselves and output their
count numbers. A state detecting counter (9) causes a state variable to
go up/down according to whether a signal (S5) and a signal (S7) agree
with each other. When the state variable is at its lowest value and the sync number
encoding portion (6) was able to specify the frame number, the signal (S5)
is reflected in the signal (S7). When the state variable has reached its
highest value, the signal (S7) is reflected in a signal (S13).
