Abstract: A scroll type fluid displacement apparatus, in particular, a compressor unit is disclosed. The apparatus includes a pair of scroll members interfitting at an angular and radial offset for forming fluid pockets, each scroll member including an end plate and a spiral element. A seal plate is provided on the surface of the end plate of at least one scroll member and is movable between the surface of the end plate of the scroll member and the axial end surface of the spiral element of the other scroll member. A fluid introducing mechanism is provided on the central portion of the spiral elements for introducing a high-pressure fluid between the seal plate and the end plate to move the seal plate in the axial direction for sealing the fluid pockets more completely.