Rubber composition and styrene-based resin composition using same

The present invention provides a rubber composition that is improved in solution viscosity and cold flow. This rubber composition comprises: 20 to 90 wt% of a polybutadiene (A) that is produced by a metallocene catalyst and that satisfies the conditions that the microstructure includes 65 to 95 mol% of a cis-1,4-structure and 4 to 30 mol% of a vinyl-1,2-structure, and the weight-average molecular weight is from 2105 to 1106; and 10 to 80 wt% of a polybutadiene (B) that satisfies the conditions that the Mooney viscosity (ML1+4, 100 DEG C) is 60 or less, the 5 wt% styrene solution viscosity (St-cp) is from 20 to 110, and St-cp/ML1+4 is from 1.0 to 2.5. The sum of the polybutadiene (A) and the polybutadiene (B) is from 60 to 100 wt%.