Developing speaking proficiency in English has been highly demanded in the field of English education in Japan; however, teaching speaking in academic settings is difficult because of its complex nature. Many Japanese high school students cannot organize their spoken production coherently because they have not been explicitly taught how to meet the expectation of particular contexts or genres. Research on genre-based instruction has shown its effectiveness in the development of reading, writing, and listening skills; however, investigations of genre-based instruction have not been fully applied to the teaching of speaking. The primary purpose of this study was to investigate the effectiveness of genre-based instruction to develop the academic speaking ability of Japanese high school students through a one-year longitudinal study. The effectiveness of genre-based instruction was assessed by focusing on the development of a macro-genre, academic monologic speech, and three micro-genres—procedure, definition, and causation—for within-group assessment as well as one oral summary of a research project micro-genre for between-group assessment. The research design was a multistage intervention mixed-method approach with qualitative data gathered after the experiment. Three analytical techniques were employed: (a) multi-faceted Rasch measurement (MFRM) was used to assess the extent to which the participants’ performance improved quantitatively, (b) descriptive analyses were used to investigate frequency changes in the use of target lexis, and (c) genre analysis was used to analyze how the discourse structure of the target genres changed qualitatively. The results indicated that genre-based instruction led to improvements in the participants’ speaking ability. The findings from the analysis of the three micro-genres—procedure, definition, causation—revealed statistically significant differences between the pretest and the posttest speeches in the procedure and causation micro-genres. A descriptive analysis also revealed the increases in the use of the target lexis in the micro-genres. A genre analysis of the three micro- genres illustrated how the schematic and rhetorical structure of the participants’ speech changed to meet the genre expectations of the target micro- and macro-genre. The analysis of the oral summary of a research project micro-genre demonstrated the effectiveness of genre-based instruction, as the experimental group outperformed the comparison group. This result was supported by the MFRM results, the descriptive analysis of lexis, and quantitative and qualitative genre analyses. The external validity analysis using the TOEIC Speaking Test also confirmed the effectiveness of genre-based instruction. In sum, the results provide evidence that genre-based instruction can improve Japanese high school students’ speaking ability. / Teaching & Learning