PURPOSE: To secure a stable lock state even when an oscillating tank receives vibration and acceleration in association with the oscillation by disposing a rotary operating shaft, rotatory-operable between an open position and a lock position, on the closable cover side, and providing an engaging part that can be engaged with a locking claw, protrusively from the shaft end of the rotary operating shaft.

CONSTITUTION: An engaging part 26 decentered from the axis of a rotary operating shaft 22 is rotated around the axis of the rotary operating shaft 22 and engaged with the circular engaging face 20a of a locking hook part 20 in a locking claw 16 from below with strong pressure, thus closing a closable cover 6. In this state, the engaging plane 20C of the locking claw 16 is engaged with the plane part 26 of the engaging part 26. The rotary operating shaft 22 is therefore fixed into the lock position A by the frictional force caused by the engaging force of the engaging part 26 and circular engaging face 20a, the pressing force of the locking claw 16 to the engaging part 26, and the engagement of the engaging plane 20c with the plane part 26a.