An index value estimation device including a data acquisition unit that acquires feature amount data including a feature amount extracted from sensor data related to a motion of a foot of a user and used for estimating an index value indicating a knee state of the user, a storage unit that stores an estimation model that outputs an index value according to an input of the feature amount data, an estimation unit that estimates an output obtained by inputting the acquired feature amount data to the estimation model as an index value indicating the knee state of the user, and an output unit that outputs information about the estimated index value indicating the knee state of the user.