A pillar-shaped honeycomb structure includes inlet cells and outlet cells adjacent to at least one of the inlet cells with a partition wall interposed therebetween, wherein a ratio of an opening area (C.sub.in) of each of the inlet cells to an opening area (C.sub.out) of each of the outlet cells, a thickness (WT) of the partition wall, a cell density (CD) based on a total number of the inlet cells and the outlet cells, a depth (PD) of a sealing portion, and a distance (OF) between the midpoint of a line segment connecting the centers of gravity of the inlet cells and the outlet cells adjacent to each other with the partition wall interposed therebetween and the center of the partition wall intersected by the line segment, satisfy predetermined conditions, and furthermore, the honeycomb structure satisfies 2OFCD/(WTPD)7.

A reactor including at least one honeycomb structure having an outer peripheral wall and partition walls provided on an inner side of the outer peripheral wall, the partition walls defining a plurality of cells through which a process gas containing a capturing target gas can flow, each of the cells extending from an inflow end face to an outflow end face of the honeycomb structure. The honeycomb structure has at least one communication pore group composed of a plurality of communication pores provided at the outer peripheral wall and the partition walls so as to be positioned on one straight line L1 orthogonal to an extending direction of the cells. The communication pore group is located closer to the outflow end face side than a center C1 in the extending direction of the cells.