Particle acceleration mechanisms of protons and heavy ions in solar flares and coronal mass ejections (CMEs) are still unknown issues. Neutrons can be unique observation probes since they are hardly affected by solar and interplanetary magnetic field. However, little progress has not been made in previous neutron observations due to its insufficient sensitivity in ground-based observatories. To overcome such situation, we have launched new space mission dedicated for solar neutron observations, so-called SOlar Neutron and Gamma-ray Spectrometer (SONGS). This detector consists of multi-layered 256 plastic scintillator bars and 12×12 GAGG(Ce) scintillator array readout by SiPMs to track cosmic-ray interactions in three dimensions. In this configuration, it is sensitive to both fast neutrons (30–120 MeV) and soft gamma-rays (0.1–3 MeV). In this paper, we report on scientific motivation, instrumental details, and development status of the SONGS mission.