A motherboard mounting structure includes a first board having a first connector and a second board having a second connector electrically connected directly to the first connector of the first board. The second board includes an external terminal connectable to an external device. The external terminal is inserted (or extracted) through an external terminal insertion opening formed on an orthogonal surface of a mainframe orthogonal to a surface of the second board. A mounting board is provided and to which the first and second boards are attached. The second board is attached to the mounting board with the external terminal protruding from the external terminal insertion opening. The first and second boards are slidably attached in a prescribed direction to separate from the orthogonal surface of the mainframe.