This study aimed to evaluate the reliability of GPTZero, the most widely used AI detection tool, in a situation where the use of AI detection tools such as GPTZero and QuillBot is increasing as students frequently use AI tools. We measured the success rate of GPTZero in identifying AI-generated text by randomly submitting essays of three lengths: short (40-100 words), medium (100-350 words), and long (350-800 words). We conducted experiments using a dataset of 28 AI-generated essays and 50 human-written essays, and found that GPTZero detected most of the AI-generated essays accurately (91-100% judged as AI-generated), but some false positives occurred in the human-written essays. This suggests that GPTZero is effective in detecting purely AI-generated content, but has limitations in distinguishing it from human-written text.