  We formulate a quantum theory of the Universe based on Bayesian probability.
In this theory, the probability of the Universe is not a frequency probability,
which can be obtained by observing experimental results several times, but is a
Bayesian probability, which can define a probability of an event that occurs
just once. As an example, by applying the quantum theory of the Universe to an
action of a scalar field theory in the four dimensions as a toy model for the
theory of the Universe, we explicitly obtain the probability of the Universe
and the action of matters in the Universe.
