Respective pieces of the porous honeycomb segments (2) are stacked while interposing the adhesive layers between respective adhered surfaces (2a), and are bonded together by performing main pressurization (F1, F2) on the whole through the porous honeycomb segments (2) located on an outermost layer after stacking a predetermined number of pieces. Main pressurization force at this time is simultaneously applied to all the honeycomb segments (2) and does not act as a separating force against any of the honeycomb segments (2).