A seal structure of a high-pressure tank including: the first member in which a tank chamber is formed inside; and the second member which is fitted into the opening hole of the first member and provided with an annular groove formed on an outer peripheral surface, the seal structure includes: a seal member, having an annular shape, configured to be disposed on the tank chamber side in the groove; and a first back-up member and a second back-up member configured to be disposed further to the side opposite to the tank chamber side than the seal member in the groove, to include inclined surface parts respectively opposite to each other. The inclined surface part of the first back-up member includes a crest folding corner part in which a first low angle inclined part and a first high angle inclined part are connected to each other.