To provide an apparatus for counting living fish in water which objectively and accurately counts living fish during counting without damaging living fish during counting, does not take time even if fish are different in size, reduces a counting cost, performs laborsaving, improves operation efficiency and measurement accuracy and does not change fish during counting and an apparatus therefor.

The apparatus for counting living fish in water includes, for measuring the number of living fish, sinking an apparatus including a counting frame body equipped with a counting sensor, a passage control plate to be fixed in response to the size of living fish, a separation net frame equipped with separation nets having different sizes of meshes in response to the sizes of living fish, an illuminator attached to the lower part of the counting frame body and a collecting net that is equipped with a counting sensor and integrally incorporated with one side of the counting frame body in culture facilities such as fish preserve, etc., successively passing living fish through the separation plate of a counting machine and a counting sensor incorporated into the collecting net so as to count the number of living fish and measure the number thereof.