An electrical connection box includes: a casing that has an opening portion open upwardly, a recessed portion recessed downwardly from an upper edge portion of a side wall; and a slidable cover configured to close the recessed portion. A first thin portion is formed at a side edge portion of the recessed portion of the casing. A second thin portion is formed on the slidable cover. The first thin portion and the second thin portion overlap each other when the slidable cover is fitted into the recessed portion. The first thin portion is includes a chamfered portion where a boundary between an opening edge portion of the first thin portion and the side edge portion is chamfered. A contact portion is configured to be brought into contact with the chamfered portion.