To provide a sediment transport method in a water reservoir and its device, capable of transporting earth and sand without motive power or with low motive power, without requiring large artificial energy for transporting the deposited sediment.

An initial water flow is formed by a high pressure water pump 22 in an earth and sand transport pipe 21 arranged in the water reservoir 8 with a head provided. A sediment flow having density higher than surrounding water is formed by inputting earth and sand dredged from a chute 22 on the upper end 21a. This earth and sand flow is controlled and maintained by a pump control device 27, and the dredged earth and sand is inputted in the chute 22 to be transported to a discharge port of the other end part 21b, and the sediment flow is maintained, and the sediment can be transported without motive power or with low motive power. Thus, the deposited sediment is effectively made to flow out to save the artificial energy by effectively using a sand removal facility 4, and an effective water storage quantity can be secured, and the earth and sand can also be supplied to the downstream side.