A branch tee for sprinkler pipes, which automatically controls water stream, comprising: a branch tee body having a cylindrical shape; a first outlet formed on one end of the branch tee body and configured to connect to a pipe of a closed sprinkler head; an inlet formed on a side wall of the branch tee body and configured to connect to a water supply pipe; a second outlet formed on the other end of the branch tee body and configured to connect to a pipe of an open sprinkler head; a piston valve in the shape of a mini-cylinder located in the branch tee body configured to simultaneously control opening and closing of each of the first outlet and the second outlet while moving in a longitudinal direction of the branch tee body between a first position and a second position in the branch tee body when fire occurs; and an inner wall groove formed on an inner wall of the branch tee body between the piston valve and the first outlet, wherein the piston valve comprises: a main blocking member adjacent to the first outlet and configured to control opening and closing of the first outlet, when fire does not occur; and a sub-blocking member connected to the main blocking member adjacent to the second outlet and configured to control opening and closing of the second outlet when fire does not occur, the main blocking member and the sub-blocking member being in the form of disks arranged parallel to each other and connected by a connecting member, a projection protruding from the inner wall of the branch tee body located above the inlet configured to prevent the main blocking member from blocking the inlet; wherein the inner wall groove is located between the projection and the first outlet and spaced from the projection; wherein the mini-cylinder is composed of the disk of the main-blocking member and a side wall contacting an inner circumferential surface of the branch tee body and projecting from the side of the disk facing the first outlet, and wherein openings (17,19) are defined in the sidewall of the mini-cylinder constituting the piston valve, a spring located between the first outlet and the main blocking member; a spring stopper located on an inner circumferential surface of the branch tee body between the first outlet and the inner wall groove configured to fix an upper end of the spring; and a snap ring located above the spring stopper configured to prevent the piston valve and the spring stopper from being separated. a branch tee body having a cylindrical shape; a first outlet formed on one end of the branch tee body and configured to connect to a pipe of a closed sprinkler head; an inlet formed on a side wall of the branch tee body and configured to connect to a water supply pipe; a second outlet formed on the other end of the branch tee body and configured to connect to a pipe of an open sprinkler head; a piston valve in the shape of a mini-cylinder located in the branch tee body configured to simultaneously control opening and closing of each of the first outlet and the second outlet while moving in a longitudinal direction of the branch tee body between a first position and a second position in the branch tee body when fire occurs; and an inner wall groove formed on an inner wall of the branch tee body between the piston valve and the first outlet, wherein the piston valve comprises: a main blocking member adjacent to the first outlet and configured to control opening and closing of the first outlet, when fire does not occur; and a sub-blocking member connected to the main blocking member adjacent to the second outlet and configured to control opening and closing of the second outlet when fire does not occur, the main blocking member and the sub-blocking member being in the form of disks arranged parallel to each other and connected by a connecting member, a projection protruding from the inner wall of the branch tee body located above the inlet configured to prevent the main blocking member from blocking the inlet; wherein the inner wall groove is located between the projection and the first outlet and spaced from the projection; wherein the mini-cylinder is composed of the disk of the main-blocking member and a side wall contacting an inner circumferential surface of the branch tee body and projecting from the side of the disk facing the first outlet, and wherein openings (17,19) are defined in the sidewall of the mini-cylinder constituting the piston valve, a spring located between the first outlet and the main blocking member; a spring stopper located on an inner circumferential surface of the branch tee body between the first outlet and the inner wall groove configured to fix an upper end of the spring; and a snap ring located above the spring stopper configured to prevent the piston valve and the spring stopper from being separated.