PURPOSE: To provide an electric joint box easily applicable to automatic assembly in which the assembling performance is enhanced by avoiding incomplete assembling due to the warping of a wiring board or the molding error of a boss.

CONSTITUTION: The electric joint box 1 comprises an upper cover 2, a lower cover 3, and a wiring board 7 contained therein. A plurality of bosses 4, 5, 6 having different length are planted on the lower cover 3. The bosses 4, 5, 6 are inserted sequentially into the wiring board 7 in order to mount the wiring board 7 on the lower cover 3. Jigs are then introduced into the boss welding grooves 2A, 2B, 2C in the upper cover 2. Subsequently, the upper ends of the bosses 4, 5, 6, projecting from the grooves 2A, 2B, 2C, are thermally fused to secure the upper and lower covers 2, 3 integrally.