The present invention provides a modified conjugated diene polymer that has excellent hydrophilicity and is useful for modification of polar polymers. The present invention also provides a modified conjugated diene polymer and a polymer modifier that, when used for modification of a polar polymer, can enhance characteristics, for example, flexibility, of the composition including the polymer and also offer excellent processability and shaping properties. The present invention further provides a polymer composition including such a conjugated diene polymer. A polymer modifier (A) includes a modified conjugated diene polymer (A1) having a hydrophilic group having a polyethylene glycol structure.