The phase separation of hydroxypropylcellulose (HPC) in a mixed solvent of glycerol and water was investigated by an elongational flow birefringence method. In the one-phase region, the elongational flow birefringence had the characteristics of a typical coil-stretch transition-like pattern with a critical elongational strain rate (epsilon)over dot(c)(.)(epsilon)over dot(c) increased monotonously with temperature, but in the vicinity of the phase-separation point, (epsilon)over dot(c) began to decrease even in the one-phase region. In the two-phase region, the flow-induced birefringence pattern contained both a rigid rod-like response and the coil-stretch transition-like response of a flexible polymer. The appearance of the rod-like birefringence pattern indicates the association of HPC chains to form a precursor of the liquid-crystalline phase formation.