A design assistance device, includes: a data acquisition unit acquiring performance data including a design parameter group and an observation value of a characteristic item; a model construction unit constructing a prediction model for predicting the observation value as a probability distribution, on the basis of the design parameter group; an acquisition function construction unit constructing a single target-oriented acquisition function having the design parameter group as input and an index value relevant to improvement of characteristics of all of the characteristic items as output, the single target-oriented acquisition function including a target achievement probability term including a total achievement probability calculated on the basis of each of the prediction models, in which target values of all of the characteristic items are achieved; and a design parameter group acquisition unit acquiring at least one design parameter group by optimization of the target-oriented acquisition function.