An interconnector-electrochemical reaction unit cell composite body includes an electrochemical reaction unit cell and an interconnector. The interconnector has a plurality of combinations of protrusions protruding in the first direction and recesses provided on a side opposite the protrusions in the first direction and being concave toward the protrusions. At least one of the combinations is configured such that, as viewed in the first direction, the bottom of the recess is located on the side toward the open end of the recess. In a section parallel to the first direction, the recess is located within a width t2 of the protrusion in the second direction orthogonal to the first direction, and the width t1 of the recess in the second direction is smaller than the width t2.