A scroll compressor comprising a casing defining an interior space, wherein the interior space is vertically compartmentalized by a housing , wherein a space above the housing defines an upper space and a space below the housing defines a lower space , a compression mechanism placed above the housing including a fixed scroll and an orbiting scroll , the fixed scroll and the orbiting scroll having a compression chamber defined therebetween , wherein the compression mechanism includes: an introduction mechanism having an introduction passage which provides communication between the compression chamber and a middle-pressure back-pressure chamber , the introduction mechanism being configured to supply, throughout a first period, a fluid of the compression chamber to the middle-pressure back-pressure chamber in back of the orbiting scroll ; and an auxiliary introduction mechanism having: an auxiliary introduction passage which provides communication between the compression chamber and the middle-pressure back-pressure chamber ; and a check valve which allows the fluid to flow from the compression chamber to the middle-pressure back-pressure chamber , and blocks the fluid from flowing from the middle-pressure back-pressure chamber toward the compression chamber , the auxiliary introduction mechanism being configured to supply, throughout a second period, the fluid of the compression chamber to the middle-pressure back-pressure chamber , and the second period including a time point prior to the first period, characterized in that the introduction passage provides communication between the middle-pressure back-pressure chamber and an outermost compression chamber of the compression chamber , the auxiliary introduction passage provides communication between the outermost compression chamber and the upper-space , and in the compression chamber , an inflow end of the auxiliary introduction passage is opened closer to a low-pressure side of the compression chamber than an inflow end of the introduction passage is. an introduction mechanism having an introduction passage which provides communication between the compression chamber and a middle-pressure back-pressure chamber , the introduction mechanism being configured to supply, throughout a first period, a fluid of the compression chamber to the middle-pressure back-pressure chamber in back of the orbiting scroll ; and an auxiliary introduction mechanism having: an auxiliary introduction passage which provides communication between the compression chamber and the middle-pressure back-pressure chamber ; and a check valve which allows the fluid to flow from the compression chamber to the middle-pressure back-pressure chamber , and blocks the fluid from flowing from the middle-pressure back-pressure chamber toward the compression chamber , the auxiliary introduction mechanism being configured to supply, throughout a second period, the fluid of the compression chamber to the middle-pressure back-pressure chamber , and the second period including a time point prior to the first period, characterized in that the introduction passage provides communication between the middle-pressure back-pressure chamber and an outermost compression chamber of the compression chamber , the auxiliary introduction passage provides communication between the outermost compression chamber and the upper-space , and in the compression chamber , an inflow end of the auxiliary introduction passage is opened closer to a low-pressure side of the compression chamber than an inflow end of the introduction passage is. <EOS>