To provide an in-pipe gas replacing method for safely and inexpensively performing replacing work even when first gas in a gas supply pipe and second gas to replace the first gas are mixed together to possibly cause ignition.

In the in-pipe gas replacing method for replacing the first gas Ga in the gas supply pipe C with the second gas Gc, a pig 8 is inserted into the pipe at one end side in a pipe longitudinal direction to partition the pipe into a first space part 9a at one end side in the pipe longitudinal direction and a second space part 9b at the other end side in the pipe longitudinal direction, inactive gas Gb is supplied to a pig-side portion of the second space part to form an inactive gas region 11, and then the pig is moved to the other end side in the pipe longitudinal direction while supplying the second gas into the first space part to push the first gas in the second space part to the outside of the pipe.