A synthetic resin-made sliding bearing comprising: a synthetic resin-made upper casing having an annular lower surface and a cylindrical side surface; a synthetic resin-made lower casing on which said upper casing is fitted and which has an annular upper surface, defining an annular space extending in a radial direction in cooperation with the annular lower surface of said upper casing, and a cylindrical side surface defining a tubular space extending in such a manner as to intersect the radial direction in cooperation with the cylindrical side surface of said upper casing; a synthetic resin-made sliding bearing member which has a thrust sliding bearing portion disposed in the annular space and a radial sliding bearing portion disposed in the tubular space with a radial inner clearance with respect to said upper casing and said lower casing; and an outer resilient seal member having an outer lip portion which is resiliently bend-deformable and is brought into contact with an inner peripheral side surface in the radial direction of said upper casing so as to allow an outer peripheral portion of the annular space to communicate with an outside between said upper casing and said lower casing and seal an outer peripheral-side clearance formed by mutually opposing surfaces of said upper casing and said lower casing, characterized in that the outer lip portion, which has a maximum diameter which is equal to or greater than the sum of the radial inner clearance and a diameter of the inner peripheral side surface in the radial direction of said upper casing at a portion of the inner peripheral side surface in the radial direction of said upper casing where the outer lip portion is in contact, is resiliently bent and is brought into contact with the inner peripheral side surface of said upper casing. a synthetic resin-made upper casing having an annular lower surface and a cylindrical side surface; a synthetic resin-made lower casing on which said upper casing is fitted and which has an annular upper surface, defining an annular space extending in a radial direction in cooperation with the annular lower surface of said upper casing, and a cylindrical side surface defining a tubular space extending in such a manner as to intersect the radial direction in cooperation with the cylindrical side surface of said upper casing; a synthetic resin-made sliding bearing member which has a thrust sliding bearing portion disposed in the annular space and a radial sliding bearing portion disposed in the tubular space with a radial inner clearance with respect to said upper casing and said lower casing; and an outer resilient seal member having an outer lip portion which is resiliently bend-deformable and is brought into contact with an inner peripheral side surface in the radial direction of said upper casing so as to allow an outer peripheral portion of the annular space to communicate with an outside between said upper casing and said lower casing and seal an outer peripheral-side clearance formed by mutually opposing surfaces of said upper casing and said lower casing, characterized in that the outer lip portion, which has a maximum diameter which is equal to or greater than the sum of the radial inner clearance and a diameter of the inner peripheral side surface in the radial direction of said upper casing at a portion of the inner peripheral side surface in the radial direction of said upper casing where the outer lip portion is in contact, is resiliently bent and is brought into contact with the inner peripheral side surface of said upper casing.