Provided are: a clarifying agent composition in a powder form which has excellent fluidity; and a polyolefin-based resin composition capable of yielding a molded article having excellent clarity. The clarifying agent composition is obtained by heating a mixture in a range of 80 to 180°C, the mixture comprising: not less than 85% by mass of a benzylidene sorbitol compound represented by the following Formula (1): (wherein, R represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms; and R 1 , R 2 , R 3 and R 4 each independently represent a hydrogen atom, a halogen atom, a cyano group or an alkyl group having 1 to 4 carbon atoms); 5 to 10% by mass of tetrakis[3(3,5-di- tert -butyl-4-hydroxyphenyl)propionyloxymethyl]methane; and 6 to 9% by mass of a lubricant.