Check valves for checking reverse water flows in drainage systems sometimes make large impacts through their sudden closure caused by accidental cutoff of drain pumps. In order to characterize such phenomena and to find design criteria for impactless check valves, we observed the motion of a check valve and measured its impact acceleration and pressures in the drainage pipe, using a subscale drainage system. Two peaks of impact acceleration were observed; The first one seemed due to collision of the check valve and the valve seat, whereas the second one did due to water column separation and recombination caused by the low pressure during the contact of the valve with the seat. The former can be reduced by adding mass to the check valve and by setting an appropriate valve-seat angle. The latter possibly can be reduced by relaxing the low pressure during the contact. Subsequently, the check valve opened through the water-column recombination impacts.