Rubber composition and studless tire

A rubber composition, comprising:

• a conjugated diene polymer,
• a high-cis polybutadiene having a cis microstructure content of not less than 95% by mass,
• a polyisoprene-based rubber, and
• a silica having a nitrogen adsorption specific surface area of 40 to 400 m2/g,
• the conjugated diene polymer being obtained by polymerizing a monomer component including a conjugated diene compound and a silicon-containing vinyl compound in the presence of a polymerization initiator represented by the following formula (I):
• wherein i represents 0 or 1;
• R11 represents a C1-100 hydrocarbylene group;
• R12 and R13 each represent an optionally substituted hydrocarbyl group or a trihydrocarbylsilyl group, or R12 and R13 are bonded to each other to form a hydrocarbylene group optionally containing at least one, as a hetero atom, selected from the group consisting of a silicon atom, a nitrogen atom, and an oxygen atom;
• and M represents an alkali metal atom, to produce a copolymer, and
• then reacting a compound containing at least one of a nitrogen atom and a silicon atom with an active terminal of the copolymer,
• wherein the rubber composition comprises, based on 100% by mass of a rubber component, 1 to 45% by mass of the conjugated diene polymer, 20 to 64% by mass of the high-cis polybutadiene, and 35 to 60% by mass of the polyisoprene-based rubber, and
• the rubber composition comprises 5 to 150 parts by mass of the silica for each 100 parts by mass of the rubber component.