  The star formation rate in spiral galaxies is considered to be decreasing
continuously with time in a time scale of $10^{9}$ yr. The present star
formation activity, on the other hand, shows various degrees among galaxies. We
make a new data set of 1681 nearby spiral galaxies from available databases and
study the statistics of the present star formation activity. We analyze
far-infrared and optical B-band surface brightnesses of the H II regions and
the non-H II regions in M~31 and show that a far-infrared-to-optical B-band
flux ratio, f(FIR)/f(B), is a useful indicator of the present star formation
activity of spiral galaxies. For the sample galaxies, we make the distribution
diagram of log (f(FIR)/f(B)) versus log L(B) for each morphological type. The
distribution of f(FIR)/f(B) has a dispersion of one to two orders of magnitude
even within the same morphological type of galaxies, implying that the star
formation activities of spiral galaxies changes discontinuously in a short time
scale. Analyzing the log (f(FIR)/f(B)) versus log L(B) correlation, we suggest
that the most active star formation in galaxies does not continue longer than
10^{8} yr. We also construct a universal distribution histogram of log
(f(FIR)/f(B)) for each morphological type. The earlier-type spirals tend to
show larger variation of the present star formation activity. We discuss the
correlation between the present star formation activity and the structures and
environments of galaxies. We suggest that the short-term variation occurs
primarily due to internal processes which may change with the morphological
type of galaxies.
