  We propose a method of measurement of extragalactic magnetic fields in
observations of TeV gamma-rays from distant blazars. Multi-TeV gamma-rays are
emitted by these sources in narrow jet with opening angle of few degrees. These
primary gamma-rays interact with the infrared photon background producing
secondary electrons and positrons, which can be just slightly deflected by
extragalactic magnetic fields before they emit secondary gamma-rays via inverse
Compton scattering of cosmic microwave background photons. Secondary gamma-rays
emitted toward an observer on the Earth can be detected as extended emission
around initially point source. Energy dependent angular size of extended
emission is related to the value of extragalactic magnetic field along the line
of sight. Small magnetic fields B < 10^{-12} G in the voids of the large scale
structure can be measured in this way.
