Provided is a flexible wire to be inserted in a pipe and rotated in a high speed to remove scales in the pipe by striking the scales. The flexible wire includes a main wire line which is formed by alternately winding a central wire line with clockwise and counterclockwise wires along a longitudinal direction of the central wire line, an auxiliary wire line which is a single line having a diameter smaller than that of the main wire line and extends in a longitudinal direction of the main wire line, and an outermost wire which winds the auxiliary wire line so as to bind the auxiliary wire line and the main wire line.