Modifier for resins, adhesive compositions, and thermoplastic resin compositions

Disclosed is a modifier for resins, which contains a hydrogenated rosin ester as the active component. In the methylation product of hydrolyzate of the hydrogenated rosin ester, the component having a molecular weight of 320 accounts for at least 95wt% of the total amount of the components having molecular weights of 314 to 320, as determined by gas chromatography-mass spectrometry. Also disclosed are: a photoembrittlement inhibitor to be used in the polymer resin for adhesives, which consists of the modifier; adhesive compositions containing the photoembrittlement inhibitor; an agent for improving the melt flow properties and tight adhesion of thermoplastic resins, which consists of the modifier; and thermoplastic resin compositions containing the agent.