PURPOSE: To upgrade assembling accuracy and also facilitate fitting of a lock body by inserting a member having a gap for installing the lock body provided beforehand into a core material composition of a flush door and fixed together with the core material into one construction at the time of assembling the core material.

CONSTITUTION: A gap 4, wherein the configuration of a lock body 5 fits, is set beforehand ion a position in which the lock body 5 is inserted the lock 5 is arranged in a U shape by providing lateral crosspieces 3, 3 and a vertical crosspiece 2 in the direction of upper, lower and backside of the lock 5, thus making it a hollow construction using boards and the front direction of the lock 5 is composed of a vertical frame 1. When a door core material is assembled, these members are inserted into the lock fitting position and a core material of one united construction is formed. After cutting out the left side portion of a flush door 20 in the size of a front panel 5a of the lock body 5 in compliance with the lock 5 attaching position on the flush door 20, the lock body 5 is inserted into the opening 4 and attached to the door 20 with attaching screws 6. An outside handle 21 and an inside handle 22 are, in turn, fitted through a square core 23 and fixed with attaching screws 24. Thus, the lock body 5 fitting process can be simplified.