A work machine in which a work attachment is attached to a base machine, the work attachment having: a boom attached to the base machine so as to be raised and lowered; an inter-boom attached to a distal end of the boom so as to be rotatable about a horizontal axis; an arm attached to a distal end of the inter-boom so as to be rotatable about the horizontal axis; a work device attached to a distal end of the arm; an inter-boom cylinder provided on a belly side of the work attachment and between the boom and the inter-boom so as to rotate the inter-boom; an arm cylinder provided on the belly side of the work attachment and between the inter-boom and the arm so as to move the arm; and a work device cylinder provided in a portion of a back side of the work attachment close to the distal end of the arm, wherein the work attachment can be folded in a three-fold manner into such an attachment folded posture that the boom is disposed on the upper side of the inter-boom and the arm is disposed on the lower side of the inter-boom, the boom has a boom-side sloped surface (25b4) that is disposed on a distal end side of the boom and on a lower side of the boom in the attachment folded posture so as to be sloped downward as the boom-side sloped surface (25b4) advances toward a base side of the boom, and the inter-boom cylinder is attached between the boom-side sloped surface (25b4) and the inter-boom, the arm has an arm-side sloped surface that is disposed on a base side of the arm and on an upper side of the arm in the attachment folded posture so as to be sloped upward as the arm-side sloped surface advances toward the distal end of the arm, and the arm cylinder is attached between the arm-side sloped surface and the inter-boom, the arm has a surface facing downward when the work attachment is in the attachment folded posture, the surface including a flat arm contact surface, and an attachment surface that is positioned closer to the distal end of the arm than the arm contact surface, wherein the boom has a boom upper surface (25b2) that faces upward when the work attachment is in the attachment folded posture, and the boom upper surface (25b2) is formed to be more parallel to the arm contact surface than the boom-side sloped surface (25b4) and the arm-side sloped surface or to be parallel to the arm contact surface when the work attachment is in the attachment folded posture, characterized in that the flat arm contact surface is capable of making contact with a mounting surface, on which the work attachment is mounted, the attachment surface extends in a direction away from the mounting surface when the flat arm contact surface makes contact with the mounting surface, and the work device cylinder is attached to the attachment surface so as not to make contact with the mounting surface when the arm contact surface makes contact with the mounting surface. a boom attached to the base machine so as to be raised and lowered; an inter-boom attached to a distal end of the boom so as to be rotatable about a horizontal axis; an arm attached to a distal end of the inter-boom so as to be rotatable about the horizontal axis; a work device attached to a distal end of the arm; an inter-boom cylinder provided on a belly side of the work attachment and between the boom and the inter-boom so as to rotate the inter-boom; an arm cylinder provided on the belly side of the work attachment and between the inter-boom and the arm so as to move the arm; and a work device cylinder provided in a portion of a back side of the work attachment close to the distal end of the arm, wherein the work attachment can be folded in a three-fold manner into such an attachment folded posture that the boom is disposed on the upper side of the inter-boom and the arm is disposed on the lower side of the inter-boom, the boom has a boom-side sloped surface (25b4) that is disposed on a distal end side of the boom and on a lower side of the boom in the attachment folded posture so as to be sloped downward as the boom-side sloped surface (25b4) advances toward a base side of the boom, and the inter-boom cylinder is attached between the boom-side sloped surface (25b4) and the inter-boom, the arm has an arm-side sloped surface that is disposed on a base side of the arm and on an upper side of the arm in the attachment folded posture so as to be sloped upward as the arm-side sloped surface advances toward the distal end of the arm, and the arm cylinder is attached between the arm-side sloped surface and the inter-boom, the arm has a surface facing downward when the work attachment is in the attachment folded posture, the surface including a flat arm contact surface, and an attachment surface that is positioned closer to the distal end of the arm than the arm contact surface, wherein the boom has a boom upper surface (25b2) that faces upward when the work attachment is in the attachment folded posture, and the boom upper surface (25b2) is formed to be more parallel to the arm contact surface than the boom-side sloped surface (25b4) and the arm-side sloped surface or to be parallel to the arm contact surface when the work attachment is in the attachment folded posture, characterized in that the flat arm contact surface is capable of making contact with a mounting surface, on which the work attachment is mounted, the attachment surface extends in a direction away from the mounting surface when the flat arm contact surface makes contact with the mounting surface, and the work device cylinder is attached to the attachment surface so as not to make contact with the mounting surface when the arm contact surface makes contact with the mounting surface.