A high-pressure tank mounting structure (S1) includes: a case (22) disposed beneath a floor of a vehicle cabin and having a bottom wall (44), a peripheral wall (46) and a top wall (70); and a plurality of high-pressure tanks (18) accommodated so as to be lined up within the case (22), wherein a first hydrogen collection portion (73) that is recessed upwardly is formed at a back surface of the topwall (70) of the case (22), and a hydrogen sensing sensor (80) is disposed at the first hydrogen collection portion (73).