With daylight on 14 June, she dived, stopped her starboard propeller, and commenced hunting underwater. Less than an hour later, she sighted smoke and attempted to intercept. Her quarry, two cargo ships beat her into Sakata. That night, she moved northward again, and, at 0510 on 15 June, she sighted a passenger/cargo ship standing south past the northern end of Oga Hanto. At 0522, she fired a torpedo which hit just forward of amidships. "Koan Maru" sank in four minutes. "Sea Dog" cleared the area as small craft from the beach moved out to pick up survivors.