The present invention relates to a building type fish farming system which allows adult fish to be forwarded from a lowermost water tank after a gradual farming process that small fry are put in an uppermost water tank together with water to grow therein until the fry are grown enough to naturally move to next lower water tanks in turn, and until the fry arrive at the lowermost water tank to grow into the adult fish therein. According to the present invention, the system comprises: a first water tank (10) having circulation pipes (14) penetrating and installed on a right wall while being spaced in a vertical direction by predetermined intervals from each other a second water tank (10a) extended from the first water tank in the right direction, and having a floor wider and lower than the floor of the first water tank when viewed from the front a third water tank (10b) installed directly under the first and second water tanks, and wider than the second water tank a fourth water tank (10c) installed directly under the third water tank, and wider than the third water tank a first pathway pipe (20) slantingly installed such that an upper end thereof is coupled to the right end of the circulation pipes, and the lower end thereof is parallel to the bottom of the second water tank second and third pathway pipes (20a, 20b) installed to penetrate the bottom of the second and third water tanks such that respective upper ends of the second and third pathway pipes are exposed to the upper sections of the bottoms of the second and third water tanks, and respective lower ends thereof are isolated from the bottoms of the third and fourth water tanks a fourth pathway pipe installed to vertically penetrate the bottom of the fourth water tank such that an upper end thereof is exposed to the upper section of the bottom of the fourth water tank, and a lower end thereof extends in a lower direction a water level adjustment pipe (30) installed to be vertically coupled to respective upper
