A steam valve assembly having a main-steam stop valve, a steam control valve arranged downstream of the main-steam stop valve, and an intermediate flow path provided between, and connecting, the main-steam stop valve and the steam control valve, wherein the main-steam stop valve comprises: a first casing having a first inlet port directing horizontally, and a first outlet port directing vertically and connected to the intermediate flow path, defining a first flow path between the first inlet port and the first outlet port, and having a first valve seat arranged in the first flow path; a first valve body configured to move up and down in the first casing and to contact and leave the first valve seat, thereby to close and open the first flow path; and a first valve stem connected to the first valve body, configured to slide up and down, penetrating the first casing, and configured to move away from the first outlet port when the first flow path is opened; and the steam control valve comprises: a second casing having a second inlet port directing horizontally and connected to the intermediate flow path, and a second outlet port directing vertically, defining a second flow path between the second inlet port and the second outlet port, and having a second valve seat arranged in the second flow path; a second valve body configured to move up and down in the second casing and to contact and leave the second valve seat, thereby to close and open the second flow path; and a second valve stem connected to the second valve body, configured to slide up and down, penetrating the second casing, and configured to move away from the second outlet port when the second flow path is opened, wherein the intermediate flow path is configured to guide main steam from the first outlet port in vertically downward direction to the second inlet port in horizontal direction wherein the first outlet port and the second outlet port open downward, and the second valve stem penetrates a top part of the second casing and extends upwards, characterized in that the first valve stem penetrates a top part of the first casing and extends upwards. a first casing having a first inlet port directing horizontally, and a first outlet port directing vertically and connected to the intermediate flow path, defining a first flow path between the first inlet port and the first outlet port, and having a first valve seat arranged in the first flow path; a first valve body configured to move up and down in the first casing and to contact and leave the first valve seat, thereby to close and open the first flow path; and a first valve stem connected to the first valve body, configured to slide up and down, penetrating the first casing, and configured to move away from the first outlet port when the first flow path is opened; and a second casing having a second inlet port directing horizontally and connected to the intermediate flow path, and a second outlet port directing vertically, defining a second flow path between the second inlet port and the second outlet port, and having a second valve seat arranged in the second flow path; a second valve body configured to move up and down in the second casing and to contact and leave the second valve seat, thereby to close and open the second flow path; and a second valve stem connected to the second valve body, configured to slide up and down, penetrating the second casing, and configured to move away from the second outlet port when the second flow path is opened, wherein the intermediate flow path is configured to guide main steam from the first outlet port in vertically downward direction to the second inlet port in horizontal direction wherein the first outlet port and the second outlet port open downward, and the second valve stem penetrates a top part of the second casing and extends upwards, characterized in that the first valve stem penetrates a top part of the first casing and extends upwards.