1. An image recognition device comprising: a database in which feature data of a plurality of objects are registered, the feature data being divided into classes for each of the plurality of objects; an identification unit that identifies an unknown object by evaluating which of the feature data of a class from the classes registered in the database is most similar to unknown feature data obtained from an image of the unknown object; and a feature data registration unit that registers feature data in the database, wherein the database is capable of setting a plurality of classes to an identical object, and the feature data registration unit, in adding new feature data for a first object that is already registered in the database, sets a new class other than an existing class for the first object, and registers the new feature data in the new class, when a second object satisfying a predetermined similarity condition with respect to the new feature data exists in the database, wherein the predetermined similarity condition includes at least one of: 1) a condition that a similarity between the new feature data and feature data of a class of the second object is higher than a similarity between the new feature data and feature data of the existing class of the first object, 2) a condition that an intra-class variance of the existing class of the first object increases and an inter-class variance between the existing class of the first object and the class of the second object decreases when the new feature data is added to the existing class of the first object, and 3) a condition that distinguishability, by the identification unit, between the first object and the second object is degraded when the new feature data is added to the existing class of the first object.