An oil-water separator with a self-priming pump of high oil-water separation ability enables a mixed liquid to be introduced into the oil-water separator without causing emulsification. A separator portion separating a mixed liquid includes a rough cyclone separation portion. A main separation portion downstream of the rough separation portion performs main separation according to specific gravity. A water discharge portion connected to the main separation portion discharges water. An oil storage tank stores the oil separated from the water and overflowing the main separation portion. A self-priming liquid pump connected downstream of the separator portion and connected to the water discharge portion sucks the mixed liquid from an exterior to enable venting and discharging of air.