A working machine which comprises a boom (8, 12) thereof to be assembled, the boom (8, 12) comprising: a first boom body; and a second boom body connected to a front end part of said first boom body with pin holes (15, 16, 17, 18) and pins (13, 14) on both upper and lower sides in a state that the boom (8, 12) is horizontal, wherein said first boom body is rotatable by a first boom cylinder; wherein bosses are provided in the front end part of said first boom body so as to extend in the boom width direction, wherein said bosses, and hooks formed in a base end part of said second boom body, are arranged so as to satisfy the following conditions respectively: both said boom bodies (8, 12) are relatively rotatable around a center of said bosses by a scooping action of rotating said first boom body upward by said first boom cylinder in a state that said bosses are engaged with said hooks and the pins (13, 14) are not inserted in the pin holes (15, 16, 17, 18) such that said upper pin holes (15, 16) of said boom bodies (8, 12) are matchable with each other by said above relative rotation; characterized in that said hooks are opening downward in a state that the second boom body is horizontal, and said bosses and hooks are arranged so as to satisfy the following condition: said first boom body is rotatable upward in a state that said upper pins are inserted into said matched upper pin holes (15, 16), such that said second boom body is rotated downward around said upper pins and such that said hooks are moved away from said bosses in a state that said lower pin holes (17, 18) of said boom bodies (8, 12) are matched with each other. a first boom body; and a second boom body connected to a front end part of said first boom body with pin holes (15, 16, 17, 18) and pins (13, 14) on both upper and lower sides in a state that the boom (8, 12) is horizontal, wherein said first boom body is rotatable by a first boom cylinder; wherein bosses are provided in the front end part of said first boom body so as to extend in the boom width direction, wherein said bosses, and hooks formed in a base end part of said second boom body, are arranged so as to satisfy the following conditions respectively: both said boom bodies (8, 12) are relatively rotatable around a center of said bosses by a scooping action of rotating said first boom body upward by said first boom cylinder in a state that said bosses are engaged with said hooks and the pins (13, 14) are not inserted in the pin holes (15, 16, 17, 18) such that said upper pin holes (15, 16) of said boom bodies (8, 12) are matchable with each other by said above relative rotation; both said boom bodies (8, 12) are relatively rotatable around a center of said bosses by a scooping action of rotating said first boom body upward by said first boom cylinder in a state that said bosses are engaged with said hooks and the pins (13, 14) are not inserted in the pin holes (15, 16, 17, 18) such that said upper pin holes (15, 16) of said boom bodies (8, 12) are matchable with each other by said above relative rotation; said hooks are opening downward in a state that the second boom body is horizontal, and said bosses and hooks are arranged so as to satisfy the following condition: said first boom body is rotatable upward in a state that said upper pins are inserted into said matched upper pin holes (15, 16), such that said second boom body is rotated downward around said upper pins and such that said hooks are moved away from said bosses in a state that said lower pin holes (17, 18) of said boom bodies (8, 12) are matched with each other. said first boom body is rotatable upward in a state that said upper pins are inserted into said matched upper pin holes (15, 16), such that said second boom body is rotated downward around said upper pins and such that said hooks are moved away from said bosses in a state that said lower pin holes (17, 18) of said boom bodies (8, 12) are matched with each other.