A method for attaching a rotor of a wind turbine generator and a method for constructing a wind turbine generator are provided, with which the rotor blades can be prevented from breaking and with which the construction costs can be reduced. The present invention includes: an assembly step (S1) in which a plurality of wind-turbine rotor blades are attached to a rotor head to assemble the rotor on the ground; a wire attaching step (S2) in which a first wire is attached to a first hoisting portion provided on a rotation axis of the rotor and in which a second wire is attached to a second hoisting portion provided at a position away from the rotation axis; a hoisting step (S3) in which the rotor is hoisted with the first wire; a rotor raising step (S4) in which the first wire is extended so that the rotor is hoisted with the second wire; and a rotor attaching step (S6) in which the rotor hoisted with the second wire is attached to the nacelle.