Modified conjugated diene-based polymer and method for producing the same, and modified conjugated diene-based polymer composition

A method for producing a modified conjugated diene-based polymer comprising a nitrogen atom, the method comprising steps of:

• polymerizing at least a conjugated diene compound in a presence of an organolithium compound having at least one nitrogen atom in a molecule to obtain a conjugated diene-based polymer;
• and
• modifying the conjugated diene-based polymer with a modifier having four or more alkoxy groups bonding to a silyl group and a tertiary amino group in one molecule,
• wherein, in the modification step, a content of the conjugated diene compound is 100 ppm by mass or more and 50000 ppm by mass or less based on a total amount of a monomer and a polymer,
• wherein the modified conjugated diene-based polymer comprising a nitrogen atom has a content of the nitrogen atom of 25 ppm by mass or more based on a total amount of the modified conjugated diene-based polymer,
• a Mooney stress-relaxation rate measured at 110° C. of 0.45 or less,
• a modification percentage of 75% by mass or more, and
• a ratio of a first weight-average molecular weight to a first number-average molecular weight of 1.50 or more and 3.50 or less.