A fuel vapor leakage detection device performs a fuel vapor leakage detection method to detect a clogging of passages in an evaporated fuel processing system by (i) switching a switching valve after measuring a first reference pressure, (ii) determining whether a detected pressure equals atmospheric pressure after opening a purge valve, (iii) recording the detected pressure after closing the purge valve, and (iv) switching the switching valve to depressurize an atmospheric system and to determine whether a current detection value of the pressure sensor is the same as the first reference pressure. Then, a second reference pressure is measured for comparison with the first reference pressure, to determine whether the evaporation system is clogged. In such manner, leakage and the clogging of the passages in an evaporated fuel processing system may be detected.