PURPOSE: To prevent mis-insertion of a printed board as well as malfunction or failure by providing a printed board with a mis-insertion preventive zone which blocks insertion into positions other than normal insertion positions of a printed board insertion zone.

CONSTITUTION: A connection structure 1 of printed boards is as follows: a plurality of first to fifth option card type I printed boards 4, 5, 6, 7, 8 are inserted one after another into an option printed board insertion zone 3 provided in a mainframe 2, and these printed boards 4-8 are connected to a mother board 9 in the mainframe 2. Adjacent planes of first and second printed boards 4,5 and a panel 14 are provided with first and second mis-insertion preventive recess and projection 26,27. For example, inserting the first printed board 4 into a normal position of the printed board insertion zone 3 makes the indication of mis-insertion because the first recess 26 and the projection 29 do not fit with each other in the case where the second printed board 6 is inserted adjacent to the first printed board 4 by mistake.