To provide a grinder capable of grinding/removing stains on pipe inner/outer surfaces of a used plastic pipe.

The grinder for the used plastic pipe comprises a grinding chamber 12 storing the used plastic pipe so as to be freely take in/out and provided with a pipe taking in/out port 14, a pair of two lower rolls 2A, 2B arranged in a line in the grinding chamber 12 and including at least one grinding roll, an upper grinding roll 4 inserted in a processed pipe at an upper close position to both lower rolls 2A, 2B, and a upward/downward moving device 7 moving the upper grinding roll 4 upward/downward. The upper grinding roll 4 is supported in a back side cantilever state wherein an end portion in the pipe taking in/out port side is opened, so that both inner/outer surfaces of a ground plastic pipe are simultaneously ground by the upper and lower grinding rolls 4, 2A, 2B.