Protein aggregation causes intractable diseases such as Alzheimer’s disease. In these diseases, crystal-like protein aggregates, so-called amyloid fibrils, form in vivo and induce severe malfunction of biological tissues. Amyloid fibril formation comprises primary nucleation from a supersaturated solution of protein monomers and subsequent fibril growth. Within them, primary nucleation possesses an extremely high energy barrier, resulting in a long induction time for nucleation, typically over a decade in vivo and several days even in a test tube. Our previous study (1) revealed that ultrasonic irradiation to supersaturated protein solution drastically accelerates amyloid formation because of the effects of ultrasonic cavitation, which is applicable to clinical diagnosis (2). However, the degree of acceleration has not been controlled yet. Furthermore, the reproducibility of the induction time for nucleation highly depends on ultrasonic-irradiation conditions and the degree of supersaturation of initial protein solutions. Herein, we systematically investigate the optimized condition to accurately control amyloid formation using an originally developed sonoreactor (3) and discuss the underlying mechanism governing ultrasonic induction of amyloid formation. (1) K. Nakajima et al., Ultrason. Sonochem. 36, 206-211 (2017). (2) K. Nakajima et al., Nat. Commun. 13, 5689 (2022). (3) K. Nakajima et al., Ultrason. Sonochem. 73 105508 (2022).