Flow of mercury from a liquid-heavy-metal inflow port toward an inner forward end of a container body is rectified by a plurality of incoming-passage guide vanes in a liquid-heavy-metal incoming passage. Flow of the mercury from the forward end of the container body toward a liquid-heavy-metal outflow port is rectified by a plurality of return-passage guide vanes in a liquid-heavy-metal return passage. As a result, occurrence of stagnation and/or recirculation flows of the mercury in the container body is suppressed and a steady and highly uniform stream of the mercury is formed throughout in the container body. The container body is covered with a container outer shell to prevent any leakage of the mercury to outside due to a damage of the container body.