A locking structure for releasably locking a covering element in a closed position on a unit housing. The locking structure includes a resiliently engaging member integrally formed on the covering element, and a supporting member pivotably connected to the covering element. The resiliently engaging member includes a pair of resilient first supports projecting from the covering element, a bar connecting the respective free ends of the resilient first supports with each other and a pair of projections formed on the bar to be releasably engaged with the housing. The supporting member includes a base plate pivotably connected to the covering element and a second support extending from the base plate to be releasably engaged with the resiliently engaging member. In a locked position of the locking structure, the bar is stably supported by the second support so that the projections are kept in engagement with the housing.