A loader work machine including a vehicle body, and a working implement having booms and boom cylinders and mounting frame units for attaching the working implement to a front portion of the vehicle body. Each of the mounting frame units includes a mounting plate fixed to the vehicle body, a support base projecting laterally outward from the mounting plate, a main frame erected on the support base, and a side frame for pivotally supporting proximal ends of one of the booms and one of the boom cylinders. The side frame includes right and left side plates and a connecting element for interconnecting the right and left side plates. The side frame is detachably attached to the main frame by engaging an engaging member disposed in a lower portion of the side frame with an engageable member of the main frame, and connecting a connecting member formed on the side frame to a connectable member formed on the main frame. The main frame is formed as a one-piece plate insertable between the right and left side plates.