An oral cavity insertion tool (1) has a flat base part (10), which includes a pressing piece (30) that is tabular and folded back forward at the upper end, a pressing piece (40) that is tabular and folded back forward at the lower end and, and an air hole (13) that penetrates in forward and backward directions, a backward projection part, which includes an opposing wall part (21) that projects backward from the center of the base part (10), provides a gap in which upper teeth can be inserted between the opposing wall part (21) and the base part (10), and faces the base part (10), and an opposing wall part (81) that provides a gap in which lower teeth can be inserted between the opposing wall part (81) and the base part (10), and faces the base part (10), and a lingual frenotomy pressing plate (50), which is tabular and provided to extend downward and backward from the rear end of the backward projection part, and the lingual frenotomy pressing plate (50) has a pressing part (51) that curves to curl up, towards the back, at the rear end.
