To provide a method for detecting ground fault accidents of an underground branch transmission line and an apparatus which are easy to install and with which whether the ground fault accident is generated to a main track of the underground transmission line or which of branch lines the accident is generated can be specified in a short time, and therefore the accident can be speedily restored.

Current transformers CT1-CT6 are set to both of end parts of every predetermined section of main tracks A and B of an underground transmission line having a branch track C branching from the main tracks A and B. Outputs of both current transformers are sent to a main track detector 6, where a difference current of outputs of both current transformers is detected. Current transformers CT7-CT12 are also set to both of end parts of every predetermined section of the branch track. Outputs of the current transformers at the branch side are separated to two outputs, and one outputs are sent to the main track detector 6 to prohibit the operation of the main track detector 6. The other outputs are sent to a branch track detector 7, where a difference current of outputs of both current transformers is detected. A ground fault accident at the main track side and a ground fault accident at the branch track side are distinguished by which of the main track detector 6 and the branch track detector 7 detects.