In a method of manufacturing multiple kinds of products in an arbitrarily selected order in one manufacturing apparatus, the rate of operation of the welding robots to be disposed in each of assembly lines for each of parts is improved, the cost for equipment investment is reduced, and the time required to introduce a new kind of motor vehicle into the manufacturing apparatus is shortened, whereby the productivity is improved. The manufacturing apparatus is made up of: a plurality of part assembly lines for assembling a plurality of parts; a combining station for tack-welding these parts to thereby assemble a product; and a reinforce-welding line for reinforce-welding the product. The number of welding points for welding each of the parts in the respective assembly lines is made equal to one another for all kinds of vehicles. Those welding points of each of the parts which fall short of a required number of welding points are welded in the reinforce-welding line.