Rubber composition and tire using the same as well as modified conjugated diene-based polymer and method for producing the same

A method of producing a modified conjugated diene-based polymer, characterized by comprising:

• (1) a step of reacting a conjugated diene-based polymer having a cis-1,4 bond content of not less than 90% and a vinyl bond content of not more than 1.2% and an active terminal with a compound X having a functional group A indicating a reactivity to the active terminal and at least one reactive functional group B (provided that the functional group A and the functional group B may be same) to obtain a primary modified conjugated diene-based polymer;
• and
• (2) a step of reacting the primary modified conjugated diene-based polymer with a compound Y having a functional group C indicating a reactivity to the reactive functional group B and at least one primary amino group or protected primary amino group (provided that the functional group C may be the primary amino group or the protected primary amino group) to obtain a secondary modified conjugated diene-based polymer.