We have performed an experiment on near threshold double ionization of helium by 106eV electron impact with an improved reaction microscope. In this experiment the momenta of three particles after ionization were measured, and the information on correlation of emitted electrons was obtained. Detailed descriptions of the experimental setup and the methods of reconstruction of electron momentum were given. We focused on the analysis of momentum and energy distributions and the angular correlation of the emitted electrons. The experimental results were compared with Wannier's prediction, and it was found that the experimental results showed some characteristic features predicted by Wannier theory.