An object of the invention is to provide a decomposing system for polyisocyanate residues that is capable of suppressing reaction of polyisocyanate residues with high temperature and high pressure water to allow smooth start-up of the operation, and a start-up method for the decomposing system for the polyisocyanate residues. The decomposing system is used for hydrolyzing the polyisocyanate residues to polyamine using high temperature and high pressure water, comprising a hydrolyzer, a water feed pipe connected to the hydrolyzer, a residual feed pipe connected to the water feed pipe, a solvent feed line for filling an organic solvent in a solvent filling portion of the residual feed pipe, and a solvent draining line. Upon start-up of the operation, the organic solvent is previously filled in the solvent filling portion via the solvent feed line and the solvent draining line, first, and then, the high temperature and high pressure water is fed from the water feed pipe to the hydrolyzer. Then, the organic solvent is fed from the residual feed pipe to the water feed pipe, first, and then, the polyisocyanate residues are fed thereto.