Hi Nanako, A cavity is more commonly used. Here are some examples: I have a cavity. My tooth hurts, maybe I have a cavity.

Hi Nanako, As the others have said, it is more common to say "I have a cavity" than to say "I have a decayed tooth." I think the main difference is that "decayed tooth" gives a less pleasant image, and is longer to say than "cavity." People dislike thinking about their tooth decay, so cavity sounds cleaner and less uncomfortable to say in daily conversation. Finally, I think to say "decayed tooth" adds a focus on the "decay" of the tooth, because it is less common than "cavity" Hope this helps!

"dental caries" is the scientific term, whereas "cavity" and "tooth decay" are more commonly used in daily conversation. As Samantha said, the former is slightly more frequent, but not that much. You can check it out yourself by googling both terms and comparing the amount of results given.