A high-density fine bubble-containing liquid producing method comprising the steps of: a) producing initial liquid containing fine bubbles, wherein fine bubbles are defined as bubbles having diameters of less than 1 μm, said initial liquid does not substantially include bubbles having diameters of less than 60 nm; b) guiding said initial liquid through a first intake passage to a first filter, through which fine bubbles having small diameters pass but not fine bubbles having large diameters larger than 200 nm, said fine bubbles having large diameters flowing to an unwanted bubble exhaust passage; c) guiding said initial liquid containing said fine bubbles having small diameters that have passed through the first filter through a second intake passage to a second filter having a smaller filtration size than said first filter, said second filter does not pass fine bubbles having diameters larger than 60 nm, wherein most of said fine bubbles having small diameters do not pass through said second filter and substantially only the liquid passes, part of the liquid passing through said second filter, and the remainder of the liquid being guided along with the fine bubbles having small diameters to a fine bubble-containing liquid discharge; and d) discharging said remainder of the liquid containing said fine bubbles having small diameters through said fine bubble-containing liquid discharge to obtain a high-density fine bubble-containing liquid, wherein the density of bubbles in the high-density fine bubble-containing liquid is measured at predetermined intervals, and a degree of opening of a throttle valve which is provided on said fine bubble-containing liquid discharge is adjusted on the basis of the measurement result of said density of bubbles to control a flow rate of liquid passing through said second filter in said step c) so as to allow the density of fine bubbles in the high-density fine bubble-containing liquid to be controlled to a target density. a) producing initial liquid containing fine bubbles, wherein fine bubbles are defined as bubbles having diameters of less than 1 μm, said initial liquid does not substantially include bubbles having diameters of less than 60 nm; b) guiding said initial liquid through a first intake passage to a first filter, through which fine bubbles having small diameters pass but not fine bubbles having large diameters larger than 200 nm, said fine bubbles having large diameters flowing to an unwanted bubble exhaust passage; c) guiding said initial liquid containing said fine bubbles having small diameters that have passed through the first filter through a second intake passage to a second filter having a smaller filtration size than said first filter, said second filter does not pass fine bubbles having diameters larger than 60 nm, wherein most of said fine bubbles having small diameters do not pass through said second filter and substantially only the liquid passes, part of the liquid passing through said second filter, and the remainder of the liquid being guided along with the fine bubbles having small diameters to a fine bubble-containing liquid discharge; and d) discharging said remainder of the liquid containing said fine bubbles having small diameters through said fine bubble-containing liquid discharge to obtain a high-density fine bubble-containing liquid,