An electrical connection box has upper and lower casings (10, 11) for accommodating circuitry. A surrounding wall (10a) of the upper casing (10) fits inside a surrounding wall (11a) of the lower casing (11). Engaging locking claws (10b, 11b) project from the outer surface of the surrounding wall (10a) of the upper casing (10) and from the inner surface of the surrounding wall (11a) of the lower casing (11). An upper wall of the upper casing (10) is inclined down from a center portion to three peripheral edges. A drainage path (13) is defined by a clearance between the surrounding walls (10a, 11a) of the upper and lower casings (10, 11). A bottom wall of the lower casing (11) has drainage holes (14) at the bottom end of the drainage path (13).