PURPOSE: To provide a directional coupler for an NRD guide in order to facilitate the gap adjustment and the positioning and also to reduce the occupied area of the coupler.

CONSTITUTION: A linear part 7 is inserted into a part where the distance is smallest in reference to the connection between the main and secondary lines which construct a directional coupler. Thus the connection part of both lines is equal to the part 7 that includes even a line side face. Then the bend having the smallest curvature radius is equally divided into (n) parts among those bends that are designed as the bends having no loss. These bends that are equally fractionated into (n) pieces are all continuously arranged again in a single-stroke writing way so that a curved line is formed. When a signal is inputted at the input end of the curved line in an LSM mode, a curve part 4 is formed for the branch part secondary line of a coupler line in a range covering the input end of the curved line through a single point where the signal mode is reset in an LSM mode on the curved line.