A method of controlling a cooling device cooling a heat source, said cooling device including a compressor for circulating a coolant, a heat exchanger performing heat exchange between said coolant and outside air, a cooling unit cooling said heat source using said coolant, a liquid storage container storing said coolant in a liquid state condensed by said heat exchanger, a first passage (21, 22, 34, 36, 24 to 27) through which said coolant discharged from said compressor flows into said cooling unit through said heat exchanger and said liquid storage container, characterized in that said cooling unit is disposed below said heat exchanger in the vertical direction, and said cooling device further includes: a second passage (21, 22, 34, 36a, 51) through which said coolant is circulated between said heat exchanger and said cooling unit without applying motive power of the compressor, wherein said coolant heated and evaporated in said cooling unit being cooled and condensed in said heat exchanger and turning into a liquid coolant and returning to said cooling unit by the action of gravity when said compressor is stopped, and a switching valve switching between communication of said first passage (21, 22, 34, 36, 24 to 27) and communication of said second passage (21, 22, 34, 36a, 51), said method comprising the steps of: blocking said first passage (21, 22, 34, 36, 24 to 27) from a state of allowing communication of said first passage (21, 22, 34, 36, 24 to 27) and blocking said second passage (21, 22, 34, 36a, 51); after said step of blocking said first passage (21, 22, 34, 36, 24 to 27), increasing an amount of said coolant in the liquid state stored in said liquid storage container; and after said step of increasing an amount of said coolant, allowing communication of said second passage (21, 22, 34, 36a, 51) while maintaining blocking of said first passage (21, 22, 34, 36, 24 to 27). a compressor for circulating a coolant, a heat exchanger performing heat exchange between said coolant and outside air, a cooling unit cooling said heat source using said coolant, a liquid storage container storing said coolant in a liquid state condensed by said heat exchanger, a first passage (21, 22, 34, 36, 24 to 27) through which said coolant discharged from said compressor flows into said cooling unit through said heat exchanger and said liquid storage container, characterized in that said cooling unit is disposed below said heat exchanger in the vertical direction, and said cooling device further includes: a second passage (21, 22, 34, 36a, 51) through which said coolant is circulated between said heat exchanger and said cooling unit without applying motive power of the compressor, wherein said coolant heated and evaporated in said cooling unit being cooled and condensed in said heat exchanger and turning into a liquid coolant and returning to said cooling unit by the action of gravity when said compressor is stopped, and a switching valve switching between communication of said first passage (21, 22, 34, 36, 24 to 27) and communication of said second passage (21, 22, 34, 36a, 51), blocking said first passage (21, 22, 34, 36, 24 to 27) from a state of allowing communication of said first passage (21, 22, 34, 36, 24 to 27) and blocking said second passage (21, 22, 34, 36a, 51); after said step of blocking said first passage (21, 22, 34, 36, 24 to 27), increasing an amount of said coolant in the liquid state stored in said liquid storage container; and after said step of increasing an amount of said coolant, allowing communication of said second passage (21, 22, 34, 36a, 51) while maintaining blocking of said first passage (21, 22, 34, 36, 24 to 27).