An excavator comprising a swing type front working mechanism (7) with a boom (8) that includes a lower boom member (8L) connected at its foot pivotably via a pin (18) to brackets (12a) constituting a mount for a swing post (12). A cross-link (40) is connected pivotably via pins (41, 42) to the portion of the boom (8) which is between the brackets (12a) and an upper boom member (8U). The cross-link (40) is provided on the right side of the boom (8), not beside a cabin (6). Consequently, the cross-link (40) is put in a cantilever-like condition, and a partial load occurs on the boom (8), so that stress becomes higher on the side portion of the boom (8) on which the cross-link (40) is provided to cause lateral bending stress to be exerted on the lower boom member (8L). In order to lessen the lateral bending stress, a lateral bending load lessening means is formed which is adapted to shift the position of a boom driving hydraulic cylinder (43) from a position on an axis (A1) of the lower boom (8L) to a position on the opposite side of the cabin (6).