A water current power generating device for generating electric power from water current power, the device being characterized by comprising: an elongated sway vibrating body which is supported on its both ends and adapted to bend in a bow shape in a downstream direction in response to a fluid force exerted on its surface by a water current, to sway in such a bending state in a manner to rotate in one direction and a reverse direction thereto around a virtual straight line linking said both ends, and to generate a periodically varying tension in response to a deflection angle which said sway vibrating body forms with a direction of said water current; and at least one power converting unit for converting electro-magnetically or piezoelectrically said tension transmitted from said sway vibrating body into the electric power, wherein said sway vibrating body is made up of a string member bending like a bow in response to the fluid force exerted on its surface, and when said deflection angle of said sway vibrating body is "zero", and therefore said sway vibrating body reaches a bending state to a maximum, said tension transmitted from said sway vibrating body to said at least one power converting unit reaches its maximum, whereby output voltage from said at least one power converting unit reaches its maximum, whereas, when said deflection angle of said sway vibrating body reaches its maximum, and therefore said sway vibrating body reaches a bending state to a minimum, said tension transmitted from said sway vibrating body to said at least one power converting unit reaches its minimum, whereby output voltage from said at least one power converting unit reaches its minimum, thus alternating current power being generated. an elongated sway vibrating body which is supported on its both ends and adapted to bend in a bow shape in a downstream direction in response to a fluid force exerted on its surface by a water current, to sway in such a bending state in a manner to rotate in one direction and a reverse direction thereto around a virtual straight line linking said both ends, and to generate a periodically varying tension in response to a deflection angle which said sway vibrating body forms with a direction of said water current; and at least one power converting unit for converting electro-magnetically or piezoelectrically said tension transmitted from said sway vibrating body into the electric power, wherein said sway vibrating body is made up of a string member bending like a bow in response to the fluid force exerted on its surface, and when said deflection angle of said sway vibrating body is "zero", and therefore said sway vibrating body reaches a bending state to a maximum, said tension transmitted from said sway vibrating body to said at least one power converting unit reaches its maximum, whereby output voltage from said at least one power converting unit reaches its maximum, whereas, when said deflection angle of said sway vibrating body reaches its maximum, and therefore said sway vibrating body reaches a bending state to a minimum, said tension transmitted from said sway vibrating body to said at least one power converting unit reaches its minimum, whereby output voltage from said at least one power converting unit reaches its minimum, thus alternating current power being generated.