Box, chemical heat storage reactor, and chemical heat storage system

Powder of a heat storage material (30A, 30B) is suppressed from getting into a flow path (27A, 27B) of water vapor. A box (28, 40) is configured with a first wall portion (46) disposed between the water vapor flow path (27A, 27B) and an aggregate body of the powder, a plurality of filtration holes (50) formed in the first wall portion (46) that allow passage of water vapor from the flow path (27A, 27B) to the aggregate body while restricting the powder from penetrating into the flow path (27A, 27B); and a plurality of second wall portions (41, 42, 43, 44) disposed along a peripheral edge of the first wall portion (46) and surrounding the aggregate body. The first wall portion (46) and the second wall portions (41, 42, 43, 44) are configured by folding a sheet material (60, 160), such that at least one of either a corner portion between two of the second wall portions (41, 42, 43, 44) or a corner portion between the first wall portion (46) and one of the second wall portions (41, 42, 43, 44) is formed.