1. An emotional state determining apparatus comprising: a multifractal dimension computing section configured to compute multifractal dimensions based on one or more brain wave signals measured in one or more regions of a subject's brain, or based on one or more brain wave difference signals, each of which is obtained as a difference between two different brain wave signals in one or more sets of the two different brain wave signals that are selected from a plurality of brain wave signals measured in a plurality of regions of the subject's brain; and an emotional state determining section configured to receive data on the multifractal dimensions as inputs and determine an emotional state of the subject based on determination criteria which are determined in advance by using as reference data the one or more brain wave signals or the one or more brain wave difference signals obtained from a reference person in each of a plurality types of emotional states which are determined in advance, the emotional state determining section comprising: a storage section configured to store the determination criteria; and a determining section configured to determine one of the plurality of types of emotional states as the emotional state of the subject based on the determination criteria and the data on the multifractal dimensions, wherein: the determination criteria stored in the storage section are defined such that reference multifractal dimensions are computed based on the one or more brain wave signals or the one or more brain wave difference signals obtained as the reference data from the reference person in each of the plurality of types of emotional states and that, when the reference multifractal dimensions for each of the plurality of types of emotional states are input to the determining section, the determining section correctly determines the emotional state corresponding to the input reference multifractal dimensions; and the multifractal dimensions are generalized latent dimensions Dq 1 to Dq n respectively obtained by substituting n different values (q 1 to q n ) determined in advance for a Hurst exponent characteristic q in a generalized latent dimension Dq=1/Hq which is a reciprocal number of a generalized Hurst exponent Hq obtained from the one or more brain wave signals or the one or more brain wave difference signals, where q 1 to q n are each a positive number and n is a positive integer of 2 or more.