PURPOSE: To facilitate assembly and realize automation by using a robot by dividing a car body to an engine compartment, floor unit and a roof unit, and carrying out assembly by joining each unit after installing the functional parts onto each unit.

CONSTITUTION: An engine compartment 3 includes a dash panel 9, and a front unit 5 is equipped with a floor panel 27 as main body. Further, a roof unit 7 includes a body side 35 equipped with a variety of pillars 45 and 47 and a T bar roof 37 having a largely opened roof part. On three units 3, 5 and 7, a various kinds of functional parts are assembled previously in each independent state. The floor unit 5 is arranged at the rear part of the engine compartment 3, and the part over the floor unit 5 is covered by the roof unit 7, and the engine compartment 3, floor unit 5, and the roof unit 7 are tightening-joined each other, and easy assembly is enabled.