A lock mechanism is provided for locking inside a casing a plurality of printed circuit boards which are connected through connectors to a backplane inside the casing. The lock mechanism has an engaging member provided with a plurality of engaging pieces respectively engageable with engaged portions formed respectively in the plurality of printed circuit boards. When the engaging pieces are respectively engaged with the engaged portions, the printed circuit boards are pressed in a direction to fit the connectors of the printed circuit boards with the connectors of the backplane.