To provide a simple detection method for piping-system gas leak involved in gas filling into a tank.

A gas filling device 100 performs a pipe conduit leak detection following tank leak detection which is performed by filling extremely low temperature nitrogen gas and detection gas (hydrogen gas) into a tank T from a detection gas filling mechanism 510. In the pipe conduit leak detection, leak detection gas (residual gas) residing in the tank T when completing the tank leak detection is delivered wholly into a looped pipe conduit including the tank T, which is connected via supply/exhaust connection piping 240 to downstream side supply piping 212d downstream of a supply valve 216 located in a pipe conduit closing position, tank side supply piping 310, upstream side exhaust piping 222u upstream of an exhaust valve 224 located in the pipe conduit closing position, and tank side exhaust piping 320. Then, looped-pipe conduit leak is detected according to the transition of pressure detected by each of a supply side pressure sensor 246 and an exhaust side pressure sensor 248 in the supply/exhaust connection piping 240.