PURPOSE: To surely piece yarns by detecting the suction-catchings of the upper and lower yarns, respectively, during the piecing operation of the broken yarns, stopping the piecing operation when either of the upper and lower yarns is not detected, and subsequently again trying the piecing operation from the initial operation.

CONSTITUTION: When the breakage of a yarn is generated and pieced in a process comprising doubling the yarns Y1, Y2 supplied from two yarn-supplying packages 2, 2 with a doubler 1 and subsequently winding up the doubled yarn on a package 11, a suction mouth 15 is rotated to approach the winding package 11, suction-catches the end of the upper yarn YA and subsequently rotated downward to guide the end of the yarn to a yarn-piecing section 14, while a junction pipe 16 is rotated to the neighborhood of a yarn guide 6, catches the end of the lower yarn YB on the yarn- supplying side, guides the caught end into the piecing section 14, and subsequently subjects the ends of the upper and lower yarns YA, YB to their mutual piecing. When either one of a detection sensor S15 provided for the suction mouth 15, a pickfinding detection sensor S17 provided for the yarn guide 17, a lower yarn-picking detection sensor S16 provided for the junction pipe 16 and a yarn-diving detection sensor S18 provided for the upper yarn separator 18 is detected to be abnormal, the piecing operation is stopped and again tried from the initial process.