PURPOSE: To facilitate reliably performing addendum clearance control (seal block positioning) by seal block with use of thin side plate.

CONSTITUTION: Side plates 24, 25 are provided between the both sides on the high pressure side of gears 14, 15, a cover 3 and a mounting flange 4, seal block 26 is incorporated between the both side plates 24, 25. The side plate 24 forms plate parts 28, 29 which are divided in two directions from the central part 27 positioned between bosses 20, 21. Besides, a high pressure inlet port 30 is formed between the side plate 24 and the seal block 26. In consequence, the seal block 26 interferes together with the bosses 20, 21 and the side plates 24, 25, resulting in controlling the addendum clearance.