PURPOSE: To minimize an abrupt change in high-pressure steam flow in a continuous digester for paper pulp.

CONSTITUTION: A vertical vessel is provided with 1st and 2nd ring screens 12, 13 separated from each other in the vertical direction. Each screen is provided with ring headers 14, 15 and 1st and 2nd liquid outlets 21, 22 attached to each header and separated in the circumferential direction. The 2nd outlet 22 of the 1st screen is nearly aligned in the vertical direction with the 2nd outlet 24 of the 2nd screen similar to the alignment of the 1st outlet 21 of the 1st screen with the 1st outlet 23 of the 2nd screen. Liquid is recovered at the same time from the 1st outlet 21 of the 1st screen and the 2nd outlet 24 of the 2nd screen and then from the 2nd outlet 22 of the 1st screen and the 1st outlet 23 of the 2nd screen. The recovering procedures are continuously repeated. Each header is divided with a vertical wall into two sections having nearly equal volume. The material passing through the screen flows from the vertical wall to the outlet of the header by gravitational force along the downwardly sloping bottom of the header.