Fermi Gamma-ray Space Telescope was launched on 2008 June and has continued unbiased all-sky survey in MeV/GeV band for more than 5 years. Thanks to the high sensitivity and good spatial resolution, Fermi-LAT detected ~1000 blazars in extragalactic sky. These Fermi blazars allow us to study not only blazar physics itself (e.g, particle acceleration and energy dissipation in relativistic jets) but also Extragalactic Background Light (EBL) and Intergalactic Magnetic Field (IGMF), direct measurements of which are very difficult and almost impossible. In this talk, I will present our recent results on
1. constraints on the intensity of EBL using distant blazar PKS 0426-380 at z=1.1
2. Suzaku X-ray follow-up for GeV-faint extreme blazars and constraint on IGMF strength.
I will also discuss perspectives of how future instruments such as Astro-H and CTA will resolve the remaining issues.