A lift arm having a plate-like structure that is capable of taking a large load is realized without increasing a plate thickness and at low cost. The structure with required strength and of low cost is realized by a simple construction and achieves, at the same time, both weight lightening of a loader and sufficient load bearing performance. In a loader (2) of a working machine (1), connection sections between a cross member (176) and lift arms (171) are each formed in a shape broadening from the cross member (176) toward a lift arm (171). Further, reinforcement plates are attached to outer surfaces of a front loader (2) so as to overlap the connection sections between the cross member (176) and the lift arms (171). Contact members (171b, 171b) are attached to lower side faces at the heads of the lift arms (171) to restrict the amount of rotation of a working device installed on the heads of the lift arms (171).