PURPOSE: To keep the high efficiency of a gear pump or a motor by sufficiently weakening the rigidity of a back-up which is attached along the both outsides of a block-seal, thus preventing the block-seal to project from a gap, if any, formed between the seal-blcok and a cover or a mounting flange.

CONSTITUTION: A block-seal 25 consists of outside seal parts 26 and 27 which divide a loading area, stripe part 30 inserted into the concaved part 24 of a seal- block 22, and projection parts 31 formed on the both outside parts. A back-up 34 attached onto this side-surface is constituted of a U-shaped part 35 having an insertion groove 38 which is inserted into the concaved part 24 and into which a projection part 31 is inserted and the attached pieces 36 which are extended from the U-shaped part and whose rigidity is sufficiently weakened. Therefore, the force generated by the interference of the block-seal 25 acts onto the U- shaped part 35 through the projection part 31 of the block-seal, and the U- shaped part 35 is pushed into the concaved part 24, pulling the attached piece 36 whose rigidity is sufficiently weakened.