A curve radius estimating device (100) configured to estimate a curve radius of a lane on which a self vehicle is driving includes a first curve radius calculating part (110) configured to calculate a first curve radius on a steering angle, a second curve radius calculating part (111) configured to calculate a second curve radius on a yaw rate, and a curve radius estimating part (112) configured to estimate a curve radius by combining the first curve radius and the second curve radius at a predetermined combination ratio. The curve radius estimating part (112) changes the predetermined combination ratio depending on a vehicle speed.