PURPOSE: To precisely machine left and right round grooves by means of a single machine even when the width of a pin and that of a journal in a crakshaft are changed in every work or in the same work.

CONSTITUTION: A cutter 6 machining a round groove 3b on the left side of a pin P and a journal J is installed in one cutter head 4 in a crankshaft miller provided with plural cutter heads 4, 5, while a cutter 8 machining a round groove 3a on the right side of the pin P and the journal J is installed in the other cutter head 5, and these cutters 6, 8 are used alternately or simultaneously, and consequently, the round grooves 3a, 3b on both sides of the pin P and the journal J are machined in order. In this way, the round grooves 3a, 3b on the left and the right sides can be machined by means of a single device even when the width of the pin P and the width of the journal J are changed in every work 3 or in the same work 3, so that installation of plural machines are not required, and as a result, reduction in equipment costs and installation space can be accomplished.