The present invention relates to a polyallylamine-based high-molecular-weight polymer which is easily soluble in water and in an organic solvent and gives an excellently stable aqueous solution or organic solvent solution thereof which is suitable for use in the fields of fine chemicals such as the field of inkjet recording, and the polymer is a modified polyallylamine obtained by modifying a copolymer of N,N-dialkylallylamine and allylamine.