The purpose of this paper is to propose an effective method of teaching about igneous rock that contributes to an improvement of rock identification skills in junior high schools. A discrimination study of specimens and rock sketches was conducted using volcanic rock (andesite) and plutonic rock (diorite). The results of the study revealed the following points: The correct answer rate is lower when plutonic rock is observed alone. When volcanic rock and plutonic rock are compared, the differences in grain size are readily apparent, but it becomes somewhat difficult to tell how the grains were constructed. Mafic minerals and felsic minerals might be misidentified, as phenocryst and as groundmass in plutonic rock, respectively. The following points are considered to be important in the development of the ability to discriminate between igneous rock types. Two types of igneous rocks should be observed and compared, and instruction should be given regarding the difference between felsic minerals in plutonic rock and groundmass in volcanic rock.