The present invention provides a submerged type membrane separation device and a membrane separation method that can deal with a temporary increase of a treatment flow rate by taking advantage of the filterability of the constituent separation membranes of the device. In submerged type separation device including a membrane module having stacked therein membrane units in which flat sheet membrane elements having a separation membrane are arranged, a membrane unit having the highest filtration resistance or the highest pure-water permeation resistance is placed at the lowermost stage of the membrane module, thereby setting a high temporary filtration flux for the membrane module as a whole. This makes it possible to deal with a short-term flow rate increase.
