A scroll type fluid machine comprising: one scroll including a spiral wrap portion extending from a surface of an end plate ; another scroll which is opposed to said one scroll , including a spiral wrap portion extending from a surface of an end plate , said wrap portion of said other scroll overlapping said wrap portion of said one scroll to define a compression chamber ; a thrust receiving mechanism which is provided on a back surface of at least one of said one scroll and said other scroll , which permits a gap in a thrust direction between said end plates and said wrap portions to vary by a predetermined amount and supports said end plate in the thrust direction; a discharge port which is provided on a radially inner side of at least one of said one scroll and said other scroll , and which discharges compressed fluid which is compressed by the compression chamber ; and a pressure keeping mechanism (21; 44; 53; 63; 73; 83; 93; 103; 113) provided downstream of said discharge port for promptly elevating a pressure in said compression chamber since the start of operation so as to press said other scroll to said thrust receiving mechanism , the other scroll being provided with the thrust receiving mechanism at the rear surface thereof, characterized in that said pressure keeping mechanism has a small flow path area which does not include 0 until a pressure upstream of said pressure keeping mechanism reaches a predetermined upstream pressure after a compressing operation is started, the predetermined upstream pressure enabling to operate said one scroll stably to said other scroll , the flow path area is configured to be changeable into at least two levels according to the pressure upstream of said pressure keeping mechanism and to be permitted to be changed to a large flow path area after the pressure exceeds said predetermined upstream pressure. one scroll including a spiral wrap portion extending from a surface of an end plate ; another scroll which is opposed to said one scroll , including a spiral wrap portion extending from a surface of an end plate , said wrap portion of said other scroll overlapping said wrap portion of said one scroll to define a compression chamber ; a thrust receiving mechanism which is provided on a back surface of at least one of said one scroll and said other scroll , which permits a gap in a thrust direction between said end plates and said wrap portions to vary by a predetermined amount and supports said end plate in the thrust direction; a discharge port which is provided on a radially inner side of at least one of said one scroll and said other scroll , and which discharges compressed fluid which is compressed by the compression chamber ; and a pressure keeping mechanism (21; 44; 53; 63; 73; 83; 93; 103; 113) provided downstream of said discharge port for promptly elevating a pressure in said compression chamber since the start of operation so as to press said other scroll to said thrust receiving mechanism , the other scroll being provided with the thrust receiving mechanism at the rear surface thereof, characterized in that said pressure keeping mechanism has a small flow path area which does not include 0 until a pressure upstream of said pressure keeping mechanism reaches a predetermined upstream pressure after a compressing operation is started, the predetermined upstream pressure enabling to operate said one scroll stably to said other scroll , the flow path area is configured to be changeable into at least two levels according to the pressure upstream of said pressure keeping mechanism and to be permitted to be changed to a large flow path area after the pressure exceeds said predetermined upstream pressure. <EOS>