Each of a plurality of ceramic substrate members includes a via reaching an other main surface from one main surface. A gap is formed in each of first and second ceramic substrate members of the plurality of stacked ceramic substrate members to penetrate each of the first and second ceramic substrate members, the first ceramic substrate member being arranged at an outermost surface on one side in a stacking direction of the ceramic substrate members, the second ceramic substrate member being arranged at an outermost surface on the other side opposite to the one side in the stacking direction. At least a portion of a side surface and a bottom surface within the gap are covered with a protection layer. The protection layer is made of a material having an etching rate lower than that of the ceramic substrate members.