PURPOSE: To easily enclose a septic tank by providing a pair of holes, through which a rope can be passed and which are opposite to each other across the center of the manhole, in a position of the outside wall of the groove of the manhole periphery which is higher than the bottom of the groove and lower than the upper end of an inside wall.

CONSTITUTION: An annular groove 23 is formed at the top of a manhole frame 22 and at least a pair of holes 5 is provided in a position higher than the bottom 26 of the groove 23 and lower than the upper end of an inside wall 25. A manhole 4 is covered by a manhole lid with the groove 23 of the manhole frame 22 fitted around the rib of the manhole lid. Further, a rope 7 is passed from one of the holes 5 through the space above the manhole lid to the other hole 5 which is opposite to the former hole with the center of the manhole 4 between, and each end of the rope 7 passed through both holes 5 and turned back is held in place by a lock 8.