PURPOSE: To provide the captioned lamp less than 100W which is not separated in color when it is lighted in its horizontal attitude, by specifying the shape of a luminous lamp valve.

CONSTITUTION: When a luminous lamp bulb 1 is halved into a negative electrode side 1a and a positive electrode side 1b by a plane A-A' perpendicular, at a central point C of a lamp axis B, to the lamp axis B connecting respective inner end parts of both sealing parts 2a, 2b to each other, there is provided a relation S1<S2 for the inner surface area S1 of a luminous lamp valve 1a on the negative electrode side and that S2 of a luminous lamp valve 1b on the positive electrode side. Hereby, it is possible to prevent a color separation phenomenon in the luminous lamp from being produced even if the lamp is lighted in its horizontal attitude under voltage not reversed in its polarity, such as DC.