Pigment dispersion is manufactured by continuously flushing pigment press-cake. First and second troughs supply hydrophobic liquid organic media and the pigment wet-cake respectively at a constant rate to a twin screw co-rotating extruder having a L/D (length/diameter) value of 25 or more. The twin screw co-rotating extruder includes a feeding part where the hydrophobic liquid organic media and the pigment wet-cake are fed from the first and second troughs, a flushing part for flushing the pigment particles from water phase to form an organic medium phase, and a dehydrating part for removing water from the flushed mixture. The hydrophobic liquid organic media and the pigment wet-cake are fed to the twin screw co-rotating extruder, and the pigment press-cake is flushed and dehydrated continuously in the extruder to obtain the pigment dispersion.