In a synthetic resin-made thrust sliding bearing, a silicone grease whose base oil is a silicone oil, whose coefficient of kinematic viscosity at 25° C. is not less than 100 cSt and not more than 500,000 cSt, and which contains a thickener so as to set its worked penetration to not less than 200 and not more than 400 is interposed at sliding interfaces between, on the one hand, an upper surface of a lower annular flat plate-shaped portion defining a bottom surface of a lower annular recess and a lower surface of an upper annular flat plate-shaped portion and, on the other hand, an upper surface and a lower surface of a thrust bearing piece which are respectively brought into sliding contact with the upper and the lower surface.