An MH tank module 2 has a hollow outer shell portion 12 formed of metal. The outer shell portion 12 has a plurality of MH powder retaining chambers 22 defined by a plurality of fins 20. A plurality of MH tank modules 2 are bound together by a first tank holder 10 and a second tank holder 11. The tank holders 10, 11 are each configured by fastening and fixing first, second, third, and fourth holder components 28-31, which are separate from one another. The first to fourth holder components 28-31 each have a heat medium passage 28f-31f. The first to fourth holder components 28-31 each have recessed portions 33 each corresponding to the shape of a side portion of each MH tank module 2. The MH tank modules 2 are held individually by the corresponding recessed portions 33.