A modified chain aliphatic polyamine obtained by addition reaction of a chain aliphatic polyamine having a specific structure and an unsaturated hydrocarbon compound has a low viscosity and it provides, when used as a curing agent for epoxy resin, an epoxy resin composition which can provide an epoxy resin cured product having an excellent property. Further, a modified chain aliphatic polyamine composition obtained by addition reaction of a chain aliphatic polyamine having a specific structure and an unsaturated hydrocarbon compound followed by the removing step of unreacted chain aliphatic polyamine to reduce its amount less than 2% by weight provides, when used as a curing agent for epoxy resin, an epoxy resin composition which can provide an epoxy resin cured product having an excellent property.