To provide a honeycomb structure which prevents the displacement of positional relationship between respective through holes of respective stack members after stacking and causes little pressure loss and eliminates the blocking of cells.

The pillar honeycomb structure is formed by stacking the stack members with a plurality of through holes and forming a plurality of the cells. The stack member has an almost circular plate shape, and a flat surface part is provided at a surface on the outer circumferential side of the stack member.