To detect a hot line without any adverse effect on the light transmission characteristic of an optical fiber.

This hot-line detector has a deformation application member 10 formed out of the first comb tooth type member 6 having the forward end formed to the curvature of γ1 slidably combined with the second tooth comb type member 8 having the forward end formed to the curvature of γ2. The curvature γ2 is larger than the curvature γ1. First, gentle bending deformation is applied to one end of an optical fiber 2 by use of the comb tooth type member 6 having the curvature of γ1, and the existence of leak light at the bent part is detected with a light receiving element PD, thereby detecting whether a hot line exists or not. When the hot line is detected, the motion of the deformation application member 10 is interlocked. Also, when no leak light is detected, relatively sharp bending deformation is applied to one end of the optical fiber 2 by use of the tooth comb type member 8 having the curvature of γ2, and then a hot line is detected on the basis of whether leak light is detected at the bent part. According to this construction, a hot lie is detected under the prevention of an adverse effect on a light transmission characteristic.