PURPOSE: To make it easy to insert and detach a circuit board package and also improve the mounting efficiency of wiring patterns or electric parts.

CONSTITUTION: A structure is provided for preventing a vertical circuit board package 3 from coming off from a connector 2a a main circuit board 2 fixedly provided on the mounting surface of a rack 1. The rack 1 is provided with a pair of a basic board support part 4a having a guide groove 4a-1 for guiding and holding both side edges of the circuit board package 3 in a vertical direction and a board support 4 including a flexible engagement part 4b having a nail 4b-1 on the end and extending from the board holder 4a, which are oppositely provided vertically to the mounting surface of the rack 1. The board holder of the board support is made to guide and hold the circuit board package 3 and at the same time the flexible engagement parts are made to be displaced by a spring in the direction to go away from each other and when the circuit board package is connected with a connector, the nail is engaged with the circuit board package by the repulsion of a spring to make such engagement as to prevent the coming-off of a connector.