Wire harness installation structure

In a wire harness installation structure, a main wire (W1) of a wire harness (WH) is installed in such a manner that it is fixed to a vehicle body panel (1), and an electric connection box (2) is connected to a branch wire (W2) branching off from the main wire (W1) of the wire harness (WH), and the electric connection box (2) is mounted in a predetermined receiving position within a vehicle body. The branch wire (W2) is installed along a bottom surface (2a) of the electric connection box (2), and is connected at its distal end (S) to a portion of the bottom surface (2a) of the electric connection box (2) which is spaced a distance, larger than the shortest distance between a branching position (P) of the branch wire (W2) and the electric connection box, from the branching position (P).