To monitor and evaluate the emission-related components/systems and to determine the presence of malfunction or deterioration that can affect emissions, on-board diagnostics (OBD) is required by current regulations. With respect to the diesel particulate filter (DPF), a pressure drop across the DPF is monitored by the OBD since the pressure drop is approximately linear in particulate matter (PM) load and useful for evaluation. However, it is known that a sudden decrease in the DPF pressure drop is taken place under cold start conditions and the OBD misses the diagnosis. Although this seems to be caused by the water condensed in the exhaust pipe, no detailed mechanism has been obtained so far. The present study established an experimental system that can reproduce a rapid increase of the exhaust gas flow under cold start conditions and to control the amount of water supplied into the DPF. The sudden decrease in the DPF pressure drop was taken place by the water in this system as well. The observation of the PM cake layer with a pair of halved DPFs revealed that the decrease in the DPF pressure drop is caused by the peel-off and falling of the PM cake layer. The increase of the water supply rate made the PM cake layer thinner, so that the decrease in the DPF pressure drop became larger. In this paper, the effects of the gas temperature at the DPF inlet and the PM load on the DPF pressure drop were also discussed.