A sheet space-detecting device detects a space between a rear end of a preceding first fiber reinforced plastic sheet and a front end of a following second fiber reinforced plastic sheet in a conveyance path for fiber reinforced plastic sheets and includes: a light-detecting sensor that projects detection light on an area including the rear end of the first fiber reinforced plastic sheet, the front end of the second fiber reinforced plastic sheet, and a reference surface exposed from the space and receives reflected light thereof; and a spacer that supports the rear end of the first fiber reinforced plastic sheet and the front end of the second fiber reinforced plastic sheet in a state where the rear end and the front end are separated from the reference surface, in an area including at least a light-projected area on which the detection light is projected.