PURPOSE: To faciletate adjustement of seal gap, in runner seal gap adjusting method of pump or water-turbine etc., installing cutting tool to a hole for machining provided to the runner, by machining the seal surface of lower cover turning them.

CONSTITUTION: A seal mechanism of pump water-turbine provided with a seal gap 3A, 3B inside outside and upper portion between a crown 1A of the runner and upper cover 2, seal gap 5A, 5B inside and outside lower portion between runner band 1B and lower cover 4. In this case, a hole 11 for machining is formed in the runner band 1B formed above seal gap 5A shifting the height position every stage subsequently in circumferential direction of the band 1B, whole seal surface of lower cover 4 is allowed to machine by cutting tool 12 inserted into every hole 11. The runner 1 and tool 12 are rotated through the wire 23 by turning of winch 21, thus required machining is carried out.