  One of the main challenges for future 21 cm observations is to remove
foregrounds which are several orders of magnitude more intense than the HI
signal. We propose a new technique for removing foregrounds of the redshifted
21 cm observations. We consider multi-frequency interferometer observations. We
assume that the 21 cm signals in different frequency channels are uncorrelated
and the foreground signals change slowly as a function of frequency. When we
add the visibilities of all channels, the foreground signals increase roughly
by a factor of ~N because they are highly correlated. However, the 21 cm
signals increase by a factor of ~\sqrt{N} because the signals in different
channels contribute randomly. This enables us to obtain an accurate shape of
the foreground angular power spectrum. Then, we obtain the 21-cm power spectrum
by subtracting the foreground power spectrum obtained this way. We describe how
to obtain the average power spectrum of the 21 cm signal.
