To monitor emission-related components/systems and to evaluate the presence of malfunctioning or failures that can affect emissions, current diesel engine regulations require the use of on-board diagnostics (OBD). For diesel particulate filters (DPF), the pressure drop across the DPF is monitored by the OBD as the pressure drop is approximately linear related to the soot mass deposited in a filter. However, sudden acceleration may cause a sudden decrease in DPF pressure drop under cold start conditions. This appears to be caused by water that has condensed in the exhaust pipe, but no detailed mechanism for this decrease has been established. The present study developed an experimental apparatus that reproduces rapid increases of the exhaust gas flow under cold start conditions and enables independent control of the amount of water as well as the gas flow rate supplied to the DPF. The results show that the sudden decrease in the DPF pressure drop is caused by the water in the developed system used here. Observations of the soot cake layers in the DPF show that the decrease in the DPF pressure drop is caused by peeling-off and separation of the soot cake layer from the walls of the DPF. An increase in the water flow rate thins the soot cake layer and decreases the DPF pressure drop. Further, numerical simulation using a DPF model developed by a research group at Waseda University was also performed, and the calculated DPF pressure drop captures the changes obtained by the experiments well.