The present invention relates to a silicon-based active material-polymer composite which is secondary particles in which silicon-based particles as primary particles and conductive polymer particles having a modulus of elasticity of 10 Pa to 100 Pa at a temperature of 20° C. to 40° C. are uniformly mixed. The uniform mixture with the elastic conductive polymer particles provides for a buffering action against the stress generated by volumetric changes of the silicon-based particles.