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 (71, 72) 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 (71, 72) 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 (71, 72) 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 (71, 72) 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.