PURPOSE: To perform operations for parts assembling and so on without stopping a work on an assembling line, by transferring a truck for robot use synchronously with the work traveling on a transfer line for both of a work conveyor line and a robot mounting transfer truck installed in parallel with each other.

CONSTITUTION: In an operating robot mounting transfer truck 6, a clamp device 27 is additionally installed in the inside adjoining to a work conveying truck 4, while a pin to be clamped from in front and in the rear by each of clamp arms 36a and 36b of a clamp device 27 of the truck 6 is projectingly installed in the truck 4 which is coupled with the truck 6 upon being accurately adjusted to the specified position with the truck 6. Under this condition, the truck 6 is moved forward by drive of a motor, then also the truck 4 is synchronously transferred as one body, and an operating robot 8 on the truck 6 is set in operation by means of programming control, thus parts on an assembling part transitory mounting table of the truck 6 is automatically assembled to the work on the truck 4.