PURPOSE: To enhance the operating performance when the wireharness of a door courtesy switch is taken out.

CONSTITUTION: This invention relates to an installation structure where a door courtesy switch 20 is installed at the door opening section of a vehicle body. The opening section 24 is partitioned by a flange portion 28 where an inner panel 34 and an outer panel 36 are joined while being over-lapped, the inner panel and the outer panel. And the structure has a first space portion 3O adjacent to the flange portion, and a second space portion 32 which is partitioned by the inner panel and the outer panel while being adjacent to the first space portion. The spacing W1 of the second space portion 32 in the vehicle width direction is larger than the spacing W3 of the first space portion 30. The door courtesy switch 20 is installed onto the outer panel 36 with its part inserted into the first space portion 30.