NOVELTY - The tank (40) has a tank main portion (41) to which heat is supplied from a predetermined heat source to heat-transport medium (21). A thermal storage agent (42) is isolated from heat transport medium in mixed state, whose specific gravity is larger than heat-transport medium to mix with heat-transport medium in tank main portion. A heat exchanger (43) exchanges heat with another heat-transport medium and thermal storage agent mixed heat-transport medium through heat exchange surface in tank main portion for conveying heat to prescribed supply destination.    USE - Thermal storage tank used in solar thermal power generation system (claimed).    ADVANTAGE - The heat exchanger exchanges heat with another heat-transport medium and thermal storage agent mixed heat-transport medium through heat exchange surface in tank main portion for conveying heat to prescribed supply destination, hence the heat can be stored efficiently. The thermally stored heat can be utilized reliably. The heat transfer efficiency can be improved effectively. The highly efficient electric power generation can be implemented.    DETAILED DESCRIPTION - The heat-transport medium which flows to lower portion of tank main portion from predetermined heat source in the thermal storage mode is more expensive than another heat transport medium that flows through upper portion of tank main portion into predetermined supply destination in the thermal radiation mode. The thermal storage agent of heat-transport medium flows from predetermined heat source in thermal storage mode. The fusing point temperature is maintained between temperatures of another heat transport medium flows into predetermined supply destination in thermal radiation as latent heat storage agent. A perforated panel (44) partitions off lower portion of tank main portion, so that thermal storage agent is arranged by separating from the bottom face in the tank main portion. The perforated panel is provided with several holes (45) to spread thermal storage agent in heat-transport medium flowed in into lower portion of tank main portion, so that accumulation of thermal storage agent on the bottom face of the tank main portion is suppressed. An INDEPENDENT CLAIM is included for solar thermal power generation system (1) has a solar-heat collection apparatus (20) which condenses sunlight and recovers solar heat. An electric power generating apparatus (30) generates electric power supplied from said solar heat collection apparatus. The thermal storage tank stores the heat supplied from the solar heat collection apparatus. The heat-transport medium conveys the heat supplied from the solar-heat collection apparatus. Another heat-transport medium conveys heat to electric power generating apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the solar thermal power generation system. (Drawing includes non-English language text)    Solar thermal power generation system (1)    Solar-heat collection apparatus (20)    Heat-transport medium (21)    Electric power generating apparatus (30)    Thermal storage tank (40)    Tank main portion (41)    Thermal storage agent (42)    Heat exchanger (43)    Perforated panel (44)    Hole (45)