In a process for the synthesis of .sup.11 C-labeled methyl iodide which comprises producing .sup.11 CO.sub.2 by the irradiation of a proton beam, supplying .sup.11 CO.sub.2 gas to a collector of .sup.11 CO.sub.2 gas, bubbling .sup.11 CO.sub.2 gas into a reducing agent solution to reduce .sup.11 CO.sub.2, removing the reducing agent solution by evaporation, synthesizing .sup.11 CH.sub.3 I from the reduced product of .sup.11 CO.sub.2, and recovering .sup.11 CH.sub.3 I by distillation, the termination point of the .sup.11 CO.sub.2 gas supply, the termination point of the bubbling, the termination point of the evaporation of the reducing agent solution or the termination point of the distillation of .sup.11 CH.sub.3 I is decided by a radiation sensor provided in the vicinity of the reaction vessel or a transfer tube connecting the target box wherein .sup.11 CO.sub.2 gas is generated or a temperature sensor provided in an exhaust tube of the reaction vessel. The synthesizing time is shortened, and the yield of .sup.11 CH.sub.3 I is improved by improving the utilization of .sup.11 CO.sub.2 gas and the recovery of .sup.11 CH.sub.3 I of which the half-life is very short.