A medical treatment device (12; 12a; 12b; 12c; 12d) configured to treat and join body tissues (L1, L2), characterized by comprising: a pair of holding members (52, 54; 352, 354) each comprising respective holding surfaces (62a, 72a) which are opposed to each other and which are configured to hold the body tissues to be treated; a join condition maintenance assistance portion (92a, 94a; 162a, 164a; 192a, 194a; 202, 204, 206; 180, 212a, 214a; 180, 216a; 272, 272a; 372a, 386a) which is provided on at least one of the pair of holding members and which is configured to provide assistance so that a joined state of the body tissues is maintained; and a fluid infiltration prevention substance (224, 234; 262; 396, 398) disposed on the join condition maintenance assistance portion which prevents infiltration of the fluid to the body tissues to be treated; characterized in that: the device comprises an energy output portion (92, 94; 222, 232; 242, 252; 272; 282, 284, 286; 378, 390) which is provided on at least one of the pair of holding members and which is configured to supply energy to the body tissues held by the pair of holding members and join the body tissues; the holding surfaces are projected from a surface of the energy output portion; and peripheral tissues of body tissues to be joined are in contact with the holding surfaces when the holding members are closed, and the body tissues to be joined are in contact with the energy output portion when the holding members are closed. a pair of holding members (52, 54; 352, 354) each comprising respective holding surfaces (62a, 72a) which are opposed to each other and which are configured to hold the body tissues to be treated; a join condition maintenance assistance portion (92a, 94a; 162a, 164a; 192a, 194a; 202, 204, 206; 180, 212a, 214a; 180, 216a; 272, 272a; 372a, 386a) which is provided on at least one of the pair of holding members and which is configured to provide assistance so that a joined state of the body tissues is maintained; and a fluid infiltration prevention substance (224, 234; 262; 396, 398) disposed on the join condition maintenance assistance portion which prevents infiltration of the fluid to the body tissues to be treated; characterized in that: the device comprises an energy output portion (92, 94; 222, 232; 242, 252; 272; 282, 284, 286; 378, 390) which is provided on at least one of the pair of holding members and which is configured to supply energy to the body tissues held by the pair of holding members and join the body tissues; the holding surfaces are projected from a surface of the energy output portion; and peripheral tissues of body tissues to be joined are in contact with the holding surfaces when the holding members are closed, and the body tissues to be joined are in contact with the energy output portion when the holding members are closed. the device comprises an energy output portion (92, 94; 222, 232; 242, 252; 272; 282, 284, 286; 378, 390) which is provided on at least one of the pair of holding members and which is configured to supply energy to the body tissues held by the pair of holding members and join the body tissues; the holding surfaces are projected from a surface of the energy output portion; and peripheral tissues of body tissues to be joined are in contact with the holding surfaces when the holding members are closed, and the body tissues to be joined are in contact with the energy output portion when the holding members are closed.