This invention relates to a rubber composition being excellent in the low heat buildup and fracture properties (resistance to crack growth). and more particularly to a rubber composition, characterized by compounding 10-100 parts by mass of an inorganic filler and/or carbon black based on 100 parts by mass of a rubber component including not less than 10 mass % of a modified 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 a primary amino group. In this case, the modified conjugated diene-based polymer is obtained by (1) reacting the predetermined conjugated diene-based polymer having an active terminal with a compound having two or more predetermined functional groups and (2) further reacting the resulting product with a compound having a primary amino group.