Small-angle neutron scattering (SANS) is a very powerful techniques for nanostructure characterization. In this paper, we report the conceptual design and technical description of the 40m small angle neutron scattering (SANS) instrument which is being developed by KAIST and KAERI for installation at 30 MW HANARO research reactor. For the optimal design of 40m SANS, a series of computer simulation were performed. the Q-range of the 40m SANS instrument is 0.0005 Å⁻¹ - 1.0 Å⁻¹ which is a world top-class SANS Q-range. When the cold neutron spectrum used in the simulation is realized, the neutron flux at sample position is expected to be comparable to the current state-of-art SANS instrument in the world.