A junction box includes a base, a cover snap-fitted into the base and a ventilation valve on the cover. The ventilation valve includes a first hollow cylinder that has a first inner bore having a first thread and a bottom having an open area that includes an annular flange; a second hollow cylinder; and a waterproof air-permeable membrane. The second hollow cylinder has a second thread on an outer surface thereof, a first end, a second end and a second inner bore. The first end has an annular groove. The waterproof air-permeable membrane is disposed between the first end of the second hollow cylinder and the bottom of the first hollow cylinder. A specific distance, smaller than an axial feeding amount of the first thread when being threaded by one turn, is formed between the top of the annular flange and the top of the annular groove.