In this study, we drew up an instructional method to foster students’ ability to set the value of the independent variable, which is included in the ability to plan an experiment, and we intended to inspect the results practically. This study was conducted with the aim that students would come to know the following four factors: “0 value set as the standard”, “number of the values”, “interval of the values” and “the possibility of the range of the values”. Specifically, students were offered some examples of the different values of the independent variable that were set in the same context. The students evaluated them with regard to the four factors. This strategy was carried out in a second-grade lower secondary science class on “exploring the relationship between voltage and electric current”. We found that the students’ ability to set the value of the independent variable improved with regard to all four factors, and even after the lesson, their ability was enhanced.