To simplify the screwing operations in an installation work, wherein wiring appliances for building are attached to a number of box covers and save or reduce the lead time of the attachment operations of wiring appliances, lighting appliances, etc., in an electrical construction.

A coating margin 3 is made to protrude on a front side along the inner circumference of a center opening 2 formed in the synthetic resin molded plate 1 of a box cover. Appliance attachment blocks 4 and 4 are integrally formed on both the end parts of the coating margin 3. Rectangular recesses 5 and screw insertion through holes 6 are formed in the rear parts of the attachment blocks 4 and 4. Holding grooves 8 for rectangular flat springs 7 are formed along the inner circumference of the through holes 6 at the boundaries between the through-holes 6 and the recesses 5, to hold the outer circumferences of the rectangular flat springs 7 with the grooves 8. Two parallel cut parts 9 and 9 and facing tongues 10 and 10, to which tendencies of bending toward the inside of the rectangular recess 5 are given are formed in the center part of each flat spring 7. The facing cut ends 10' and 10' of the facing tongues 10 and 10 are retained with the screw threads 11' of an attachment screw 11 by the elastic forces of the tongue 10 and 10.