In an apparatus for diagnosing a brain function of a subject 20 by giving a load to a functional site of a living organism of the subject 20, receiving reactions emitted from the living organism of the subject 20 as an electric signal 40, 50, 60 or 70 and analyzing the electric signal 40, 50, 60 or 70, the electric signals 40, 50, 60 and 70 are a voice signal 40, an nystagmus signal 50, a pulse wave signal 60 and a gravity center swinging signal 70. The apparatus selects at least two from among them, calculates chaos theoretical indexes such as Lyapunov exponents according to a chaos theoretical technique, detects changes in the chaos theoretical indexes with passage of time, and collectively evaluates and diagnoses the brain function.