An iminodiacetate-type chelating fiber（IDA fiber）was prepared by means of radiation-induced graft polymerization of an epoxy-group-containing monomer, glycidyl methacrylate, onto a commercially available 6-nylon fiber and subsequent conversion of the epoxy group into an iminodiacetate group capable of removing radioactive strontium ions from seawater. Adsorption isotherms of the IDA fiber for three kinds of alkaline earth-metal group ions, i.e., Sr^2＋, Ca^2＋, and Mg^2＋, in artificial seawater were found to be well described by ternary-component Langmuir adsorption isotherms. We proposed a strontium removal technique for the immersion of the fiber with a divided amount rather than an undivided amount into seawater in a closed area. The reduction in the amount of fiber required for an identical removal percentage of strontium was demonstrated in a batch mode. Experimental adsorption-equilibrium data as a function of the number of divided immersions agreed well with the curves calculated from the ternary-component Langmuir adsorption isotherms and mass-balance equations for each alkaline earth-metal ion.