This invention relates to a novel polymerization initiator capable of introducing an active amino proton into a polymerization starting terminal without losing polymerization activity, and a novel modified conjugated diene polymer being excellent in the interaction with a filler and capable of improving a low heat buildup of a rubber composition, and more particularly to a polymerization initiator being a diamine compound in which one amino group is protected with a silylating agent and an active proton of the other amino group is replaced with an alkali metal or an alkaline earth metal, and a modified conjugated diene polymer which can be produced by using such a polymerization initiator and is a homopolymer of a conjugated diene compound or a copolymer of a conjugated diene compound and an aromatic vinyl compound and has a residue derived from a diamine compound at its polymerization starting terminal.