[en] Gelation of polydimethylsiloxane irradiated with 256 MeV Ar ion was examined. The insoluble residue of the irradiated polydimethylsiloxane was separated with membrane filters. The weight of the insoluble residue is proportional to the number of irradiated ions. This result indicates that gelation occurs in the high local dose region of the ion tracks. A gel string is generated in each ion track. The weight of a gel string increases in proportion to the initial molecular weight (M_n) of polydimethylsiloxane. It arises from an increase in the radius of the gel strings with increasing M_n. The gelation dose decreases with increasing M_n. The region with a dose higher than the gelation dose is found in the ion tracks. The radius of the gel strings estimated using the G-value of crosslinking for low-LET radiation and with an appropriate dose distribution in the ion tracks explains the experimental results well