To solve problems that fatigue of a worker is large, working efficiency is deteriorated, a welding quality is deteriorated, and readjustment work is frequently required because upward welding in a narrow work place can not be avoided since the whole member for constituting a steel plate casing is assembled by welding to a flange welded and fixed to a surface plate in a conventional steel plate casing assembling method.

Assembling is performed by joining an assembling frame 20 having the required width and depth to an upper casing horizontal flange 53a or a lower casing horizontal flange 53b by straddling the lower surface side of the upper casing horizontal flange 53a or the lower casing horizontal flange 53b of the steel plate casing. The assembling frame 20 and an upper casing 51 or a lower casing 52 of the steel plate casing in the middle of assembling are integrally supported or fixed still in its attitude, or is supported or fixed on the surface plate in a head and tail inverse attitude, in a vertically rising attitude, and in a horizontally rising attitude, and assembling work is performed in the downward direction and in the horizontal direction.