PURPOSE: To assure always the stable molten state of joint surfaces in a flash welding method by controlling a flash feed spee in such a way that the flash welding stage is shifted to an upsetting stage when the rate of increase in said speed attains zero.

CONSTITUTION: A flash feed speed V in flash welding increases with a flash lapse time (t), and attains an equil. state when a thorough molten state is attained. The relation between the first order differential dV/dt of the speed V and the time (t) is dV/dt=0 when the speed V attains a satd. state. The point of the time when the dV/dt=0 is attained is detected and it is judged that the thorough molten state is attained in the temp. of the joint surface, then the flash welding is shifted to an upsetting stage and the welding is completed.