A gas collecting method for collecting gas yielding from gas hydrates that exist at a seafloor or a lake floor, the method comprising: releasing a collecting membrane into water, a fixture being connected with a lower end of the collecting membrane and the collecting membrane being configured as a membrane that flares downward from a top thereof; by a position maintainer provided in the fixture, recognizing a three-dimensional position of the fixture in the water and then keeping the three-dimensional position of the fixture at a target position by an autonomous navigation thereof; and collecting the gas to a ship through a tube whose lower end is connected to the top of the collecting membrane, wherein the target position of the fixture sets: 1) the lower end of the collecting membrane at a water depth shallower than the seafloor or the lake floor and shallower than a first water depth, above the seafloor or the lake floor, where water and gas are released from gas hydrates in a solid state, based on vertical water temperature distribution obtained by a conductivity temperature depth profiler CTD, and 2) the top of the collecting membrane at a depth deeper than a second water depth where bubbles of the gas released from the gas hydrates disappear due to mixture of the gas with seawater or lakewater, the second water depth being determined by an acoustic sonar. releasing a collecting membrane into water, a fixture being connected with a lower end of the collecting membrane and the collecting membrane being configured as a membrane that flares downward from a top thereof; by a position maintainer provided in the fixture, recognizing a three-dimensional position of the fixture in the water and then keeping the three-dimensional position of the fixture at a target position by an autonomous navigation thereof; and collecting the gas to a ship through a tube whose lower end is connected to the top of the collecting membrane, wherein the target position of the fixture sets: 1) the lower end of the collecting membrane at a water depth shallower than the seafloor or the lake floor and shallower than a first water depth, above the seafloor or the lake floor, where water and gas are released from gas hydrates in a solid state, based on vertical water temperature distribution obtained by a conductivity temperature depth profiler CTD, and 2) the top of the collecting membrane at a depth deeper than a second water depth where bubbles of the gas released from the gas hydrates disappear due to mixture of the gas with seawater or lakewater, the second water depth being determined by an acoustic sonar. 1) the lower end of the collecting membrane at a water depth shallower than the seafloor or the lake floor and shallower than a first water depth, above the seafloor or the lake floor, where water and gas are released from gas hydrates in a solid state, based on vertical water temperature distribution obtained by a conductivity temperature depth profiler CTD, and 2) the top of the collecting membrane at a depth deeper than a second water depth where bubbles of the gas released from the gas hydrates disappear due to mixture of the gas with seawater or lakewater, the second water depth being determined by an acoustic sonar.