To prevent the development of cracks in a ferrite substrate for an MIC isolator.

In the MIC isolator, a thick strip conductor 2 and an ground conductor 4 are formed on the wide surface of a soft ferrite board 1 and a microstrip line having a three-terminal junction part is constituted of these three components, i.e., the ferrite board 1 and the conductors 2, 4, on the center part of the board 1. When a bias magnetic field is applied from a magnet 3 to the junction part through a spacer 5, the junction part executes non-reversible operation. One terminal of the junction part is terminated by an open stub 8 of a terminal resistor 7 and the non-reversible circuit executes isolator operation. When mechanical force such as assembling stress and heat stress is applied to the board 1, the board 1 flexibly changed its shape and absorbes the mechanical stress, so that damage is hardly generated and the microstrip line is hardly broken.