We are developing GRAINE project, a 10-MeV to 100-GeV cosmic $\gamma$-ray observation project that uses an emulsion telescope with high angular resolution (0.08$^\circ$ at 1 to 2 GeV) and polarization sensitivity, and a large aperture area ($\sim$10 m$^2$), by repeating long-duration balloon flights. Through various ground experiments, a balloon-borne experiment in 2011, and a balloon-borne experiment in 2015, the feasibility of cosmic $\gamma$-ray observations with a balloon-borne emulsion telescope was pioneered. Through scientific balloon-borne experiments, we can attempt to achieve the following: pioneering polarization observations for high-energy $\gamma$ rays from pulsars, active galactic nuclei, flares, and $\gamma$-ray bursts; direct probing of proton acceleration by $\pi^0$ feature detection and approaching an emission mechanism with a spatial structure for supernova remnants; resolving GeV $\gamma$-ray excess at the galactic center region; probing new physics beyond the Planck scale by polarization observations with high-energy $\gamma$ rays propagating over cosmological distances; observing transient sources, e.g. $\gamma$-ray bursts and flares, with high photon statistics and high sensitivity; understanding emission mechanism of giant radio pulses from pulsars by searching for a $\gamma$-ray correlation; constraining intergalactic magnetic field by searching for GeV $\gamma$-ray pair halos. Developments in scientific balloon-borne experiments are ongoing.