Using detectors, skewed conveyance of sheet media such as checks can be detected accurately regardless of whether part of the conveyed medium is folded over. The biased feed detection unit 33 of a check processing device 1 according to at least one embodiment of the invention determines the lengths L1 and L2 of the conveyed check 6 (ST31 and ST32) and the thickness of a folded part of the check 6 (ST33) at different positions perpendicular to the transportation direction based on the output of a paper thickness detector 25 and a length detector 26 disposed to the check transportation path 7. The folded length L3 is then subtracted from the difference ΔL of the detected lengths to acquire a corrected difference ΔL1 (ST35). If the corrected difference ΔL1 is less than or equal to a threshold value D, the medium is determined to be conveyed normally (ST37). If the corrected difference ΔL1 exceeds the threshold value D, the medium is determined to be skewed (ST38). Skewed conveyance of the check 6 can thus be accurately determined.