Process for producing thermoplastic resin composition

A process for producing a thermoplastic resin composition, comprising melting-kneading a thermoplastic resin and a neutralizing agent, and melting-kneading the melting-kneaded product obtained in a first step and a compound (I): wherein two R1s, two R2s, R4 and R5 each represent independently a hydrogen atom, an alkyl group etc.; two R3s each represent independently a hydrogen atom or an alkyl group; X represents a single bond, a sulfur atom or a -CHR6- group, wherein R6 represents a hydrogen atom, an alkyl group etc.; A represents an alkylene group or a *-COR7- group, wherein R7 represents a single bond or an alkylene group, and a symbol * represents a bond with an oxygen atom of a >P-O- portion; one of Y and Z represents a hydroxyl group, an alkoxy group or an aralkyloxy group, and the other of Y and Z represents a hydrogen atom, or an alkyl group.