1. An information processing device which, based on an initial estimated expression made up of a plurality of operators including an operator for performing computation of collaborative filtering which employs computation and an initial evaluation matrix made up of evaluation values indicating an evaluation for each of a plurality of users as to each of a plurality of targets, some of which targets are evaluated generates a new estimated expression to be employed for estimation of evaluation values of unevaluated targets included in said targets which have not been evaluated by said users, said information processing device comprising: estimated expression candidate generating means configured to take said initial estimated expression and generate a new estimated expression wherein a portion of said initial estimated expression is modified, and an arbitrary new estimated expression, generated by combining operators selected randomly, as estimated expression candidates; estimation result generating means configured to estimate an evaluation value as to each of said plurality of evaluation targets for each of said users for each of said estimated expression candidates based on said estimated expression candidates and said initial evaluation matrix to generate estimation results made up of predictive evaluation values that form a new evaluation matrix for each estimated expression candidate; evaluating means configured to calculate an evaluation of each of said estimated expression candidates based on said estimation results and said new evaluation matrixes; and selecting means configured to select the estimated expression candidate having the highest evaluation calculated by said evaluating means, of said estimated expression candidates, as the new estimated expression to be employed for estimation of evaluation values of said unevaluated targets.