An internal sleeve in a double-lamp socket includes a tubular socket having a through hole in middle periphery abutting a longitudinal slot in upper portion, a pair of hooks each facing a block plate on lower opposing peripheries to define a pair of gaps therebetween, a common contact plate disposed in an inlaid groove in an inner periphery, an internal sleeve disposed in an upper inlaid groove, a pair of first and second contact plates from a pair of electric wires respectively engaged within a concave of the internal sleeve and a lower inlaid groove in the socket and a pair of first and second lamps inserted into the top and the bottom of the socket each having a pair of lead-in wires respectively engaged with the common contact plate and the first and second contact plate. Wherein the first lamp has an inverse L-shaped stop plate engaged within the slot and the second lamp has a pair of L-shaped stop plates respectively engaged within the gaps. Thereby, a series circuit is constituted inside the socket.