To provide a method for producing liquid spawn of mushroom which is facilitated in work for charging liquid seed spawn into a culturing tank from a seed spawn-culturing bottle, and to provide a cap of the seed spawn-culturing bottle.

The method for producing liquid spawn of mushroom includes a seed spawn-culturing step and a main culturing step so as to increase mushroom spawns using culture liquid to produce the mushroom spawns in liquid state in large volume, wherein culturing liquid 19 of a first stage and fungi are charged in the seed spawn-culturing bottle 10 having a bottle opening 11 provided with a male thread part 12; the bottle opening 11 is blocked by screwing the cap 20 of the seed spawn-culturing bottle provided with a female thread part 22 so as to be externally fitted; a filter 25 is disposed at the cap 20 of the seed spawn-culturing bottle so as to secure air permeability to the inside of the seed spawn-culturing bottle 10; the cap 20 of the seed spawn-culturing bottle is unthreaded so that liquid seed spawn 29 is charged into the culturing tank charged with culture liquid of a second stage from the seed spawn-culturing bottle 10 by transfer of air pressure.