A non-reciprocal circuit device comprising a ground plate having pluralities of external projections, a resin member having a hole through which the ground plate is exposed, a garnet plate disposed in the hole of the resin member, a microstrip line member disposed on a main surface of the garnet plate, a partition member disposed on the microstrip line member, and a permanent magnet disposed on the partition member between a pair of metal cases in this order from bottom, at least part of the external projections of the ground plate extending through the hole of the resin member over an upper surface of the partition member, and being bent to enclose the garnet plate, the microstrip line member and the partition member.