Fractional flow reserve (FFR) is a pressure-wire-based index that is used during coronary angiography to assess the potential of a coronary stenosis to induce myocardial ischemia. Recent ESC guidelines referred to the usefulness of FFR extensively when noninvasive stress imaging is contraindicated, non-diagnostic, or unavailable. However, FFR requires additional manipulation with maximal and stable hyperemia by intravenous adenosine. More routine use of FFR for all angiographically significant stenoses would add considerable time, cost, and complexity to each PCI procedure and might also increase the risk of catheter-related complications such as coronary dissection and perforation. Although the guideline mentioned that FFR may not be useful in very high grade lesions (angiographically >90%) which always have an FFR <0.80, it have not been revealed yet proper criteria to predict FFR <0.80 obtained by angiographic parameters including degree of stenosis, lesion location and vessel size. It would be valuable to find more precise criteria available by conventional angiography for discrimination of functional stenosis in way to reduce the risk of additional procedure. For the purpose, the investigators will perform FFR in the lesions with significant stenosis (>70% diameter stenosis by visual estimation) and compare the angiographic parameters and FFR values in the study.

Inclusion Criteria:

• Patients ≥ 19 years old
• Typical angina
• Stable or unstable angina pectoris
• At least one or more major epicardial coronary arteries with significant stenosis (>70% diameter stenosis by visual estimation on angiogram) confined to the proximal portion of left anterior descending artery, left circumflex artery, or right coronary artery
• Reference vessel diameter of target lesion ≥3.0 mm
• Normal LV ejection fraction (≥50%) without wall motion abnormality by echocardiography

Exclusion Criteria:

• Previous myocardial infarction
• Previous coronary bypass graft surgery
• Cardiogenic shock
• Multiple lesions in the vessel of interest
• Contraindication or hypersensitivity to adenosine or contrast media
• Reduced coronary blood flow (TIMI flow grade <3) in the vessel of interest
• Heavy calcified (definite calcified lesions on angiogram) or extremely tortuous lesions
• Pregnant women or women with potential childbearing
• Inability to understand or read the informed content

Patients with significant lesions with more than 70% diameter stenosis at proximal part of one or more major coronary arteries, regardless of revascularization strategy.