A controller of a wind turbine for causing the wind turbine to perform rotor turning, comprising: an input part for receiving a target azimuth angle at which a rotor of the wind turbine is to be stopped; and a pitch control part configured to cause the wind turbine to perform a pitch control to stop the rotor at the target azimuth angle, wherein the pitch control part is configured to: control a pitch angle of a wind turbine blade of the wind turbine so that the rotor rotates at a predetermined rotation speed which is constant, in a first period until the rotor reaches a control-switch azimuth angle immediately before the target azimuth angle; and control the pitch angle so that a rotation speed of the rotor decreases from the predetermined rotation speed in a second period after the rotor reaches the control-switch azimuth angle and before the rotor reaches the target azimuth angle, and characterized in that the controller is configured to: a) either calculate the control-switch azimuth angle on the basis of at least one of the pitch angle in the first period or a wind-velocity in the first period, b) or retrieve the control-switch azimuth angle from a memory storing the control-switch azimuth angle in advance. an input part for receiving a target azimuth angle at which a rotor of the wind turbine is to be stopped; and a pitch control part configured to cause the wind turbine to perform a pitch control to stop the rotor at the target azimuth angle, wherein the pitch control part is configured to: control a pitch angle of a wind turbine blade of the wind turbine so that the rotor rotates at a predetermined rotation speed which is constant, in a first period until the rotor reaches a control-switch azimuth angle immediately before the target azimuth angle; and control the pitch angle so that a rotation speed of the rotor decreases from the predetermined rotation speed in a second period after the rotor reaches the control-switch azimuth angle and before the rotor reaches the target azimuth angle, and characterized in that the controller is configured to: a) either calculate the control-switch azimuth angle on the basis of at least one of the pitch angle in the first period or a wind-velocity in the first period, b) or retrieve the control-switch azimuth angle from a memory storing the control-switch azimuth angle in advance. control a pitch angle of a wind turbine blade of the wind turbine so that the rotor rotates at a predetermined rotation speed which is constant, in a first period until the rotor reaches a control-switch azimuth angle immediately before the target azimuth angle; and control the pitch angle so that a rotation speed of the rotor decreases from the predetermined rotation speed in a second period after the rotor reaches the control-switch azimuth angle and before the rotor reaches the target azimuth angle, and characterized in that the controller is configured to: a) either calculate the control-switch azimuth angle on the basis of at least one of the pitch angle in the first period or a wind-velocity in the first period, b) or retrieve the control-switch azimuth angle from a memory storing the control-switch azimuth angle in advance. a) either calculate the control-switch azimuth angle on the basis of at least one of the pitch angle in the first period or a wind-velocity in the first period, b) or retrieve the control-switch azimuth angle from a memory storing the control-switch azimuth angle in advance.