To provide a locking device for a cover of an entrance for a common conduit, etc., which is not brought to the state, in which the locking device cannot be released by the intrusion of a foreign matter, and a released state can be confirmed at a glance and which is also released and operated simply.

The locking device for the cover of the entrance for the common conduit, etc., has a fixed cylindrical body 4 fixed onto a base 5 installed to the cover 1, an internal cylinder 6, which is inserted into the fixed cylindrical body 4 and in which a male screw section is mounted at an upper end, groove sections 9 are formed to an intermediate section and a shaft section 10 is set up at a lower end, a rear cover 13 fastened at the lower end of the fixed cylindrical body 4 for bearing downward load by the internal cylinder 6 at the time of locking and setscrews 14 fitted into the groove sections 9 for limiting the movement of the internal cylinders 6 and fixed onto the fixed cylindrical body 4. The locking device consists of an O ring 12 fitted at the lower end of the male screw section, a latch 11 fastened at the lower end section of the shaft section 10 and engaged with the backing plate section 3 of a fixed wall 2 and a handle, by which a key section is inserted into a keyhole 8 formed to the top face of the male screw section and which is used for rotating and shifting the internal cylinder 6.