To provide an assembling building capable of facilitating taking-in and out of articles by a truck and constituting a lower rail part without taking time in execution of a dirt floor.

The assembling building has a front sill provided on both the sides of an entrance and exit opening part in front of a dirt floor, and an upper rail part 16 and a lower rail part 18 for moving a door closing the entrance and exit opening part, and constitutes the lower rail part 18 by a plurality of staging slide rails 32 so that it is slid along the lower rail part 18 in the state where the door 15 is suspended. A slide rail attaching member 22 is provided on a front sill 6, a lower slide rail 24 is attached to a door lower frame 23 provided on the lower part of the door 15 together with the movement of the door 15, and an upper slide rail 25 guiding the lower slide rail 24 through an intermediate slide rail 26 is attached to a slide rail attaching member 22.