  We formulate and study a quantum field theory of a microtubule, a basic
element of living cells. Following the quantum theory of consciousness by
Hameroff and Penrose, we let the system to reduce to one of the classical
states without measurement if certain conditions are
satisfied(self-reductions), and calculate the self-reduction time $\tau_N$ (the
mean interval between two successive self-reductions) of a cluster consisting
of more than $N$ neighboring tubulins (basic units composing a microtubule).
$\tau_N$ is interpreted there as an instance of the stream of consciousness. We
analyze the dependence of $\tau_N$ upon $N$ and the initial conditions, etc.
For relatively large electron hopping amplitude, $\tau_N$ obeys a power law
$\tau_N \sim N^b$, which can be explained by the percolation theory. For
sufficiently small values of the electron hopping amplitude, $\tau_N$ obeys an
exponential law, $\tau_N \sim \exp(c' N)$. By using this law, we estimate the
condition for $\tau_N $ to take realistic values $\tau_N$
\raisebox{-0.5ex}{$\stackrel{>}{\sim}$} $10^{-1}$ sec as $N$ \raisebox{-0.5ex}
{$\stackrel{>}{\sim}$} 1000.
