To provide a lock for paddy field, which allows the easy operation of an opening/closing lid and can surely prevent water leakage from between an opening part of a water passing pipe and the opening/closing lid.

The cylindrical water passing pipe 11, both the ends of which are opened, is provided. A fixing member 12 is provided inside the pipe 11. The opening/closing lid 13 is provided in such a manner as to be located outside at least one opening (opening part) 11A of the pipe 11 so as to close the opening (opening part) 11A. The fixing member 12 is arranged inside an edge of the opening (opening part) 11A. A supporting member 16 for turnably fixing the lid 13 to the fixing member 12 is provided, and the lid 13 is turnably fixed to the fixing member 12 by the supporting member 16. The lid 13 is turned by making the supporting member 16 serve as a center, so that the opening (opening part) 11A can be opened/closed.