Front mount bases (14, 15) which are located at four corner portions under a cab (11) are constituted by base plates (14A, 15A) and fitting joint projections (14B, 15B), and rear mount bases (16, 17) are constituted by base plates (16A, 17A) and fitting joint projections (16B, 17B). On the other hand, front pillars (22, 23) to be erected on the front mount bases (14, 15) are provided with notched portions (22B, 23B) for fitting engagement with the fitting joint projections (14B, 15B), respectively. Rear pillars (24, 25) to be erected on the rear mount bases (16, 17) are provided with notched portions (24A, 25A) for fitting engagement with the fitting joint projections (16B, 17B). Accordingly, a weld can be formed along and between marginal edge portions around the fitting joint projections (14B, 15B, 16B, 17B) and notched portion (22B, 23B, 24A, 25A) in such a way as to increase the welding distance and to increase the strength of the welded joint portions.