An earthquake-resistant flexible slip joint apparatus includes a joint outer part including an outer portion, an end connection portion, and an inner portion, and a slip inner part, wherein a pressurizing space is formed to be surrounded by the outer portion, the end connection portion, and the inner portion of the joint outer part, the inner portion includes a packing portion at a region thereof spaced apart from the end connection portion, the inner portion further includes an inner crimped portion and the packing portion, and an adhesion between the inner portion and the slip inner part increases due to a pressure of a conveyed fluid, the pressure being applied via the pressurizing space, and the inner crimped portion is elastically deformed when the slip inner part moves in up, down, left, and right directions, so that the conveyed fluid does not leak to the outside.