The purpose of this study is to compare the exercise capacity (6-min walking distance) under the following 4 conditions using High-flow nasal cannula (HFNC); 1. FIO2 value that the minimum SpO2 value in a 6-minute walking test (6MWT) is 86-88%, and a flow of 10 L/min 2. FIO2 value that the minimum SpO2 value in a 6MWT is 86-88%, and a flow of 40-50 L/min 3. FIO2 value that the minimum SpO2 value in a 6MWT is 92-94%, and a flow of 10 L/min 4. FIO2 value that the minimum SpO2 value in a 6MWT is 92-94%, and a flow of 40-50 L/min

In the pervious study, the investigators demonstrated that 4 weeks of training using both high FIO2 and high flow through an HFNC significantly improved the 6MWD compared with training using a 6 L/min nasal cannula. However, it was unclear whether the effects of pulmonary rehabilitation under HFNC were due to high FIO2, high flow rate, or a synergistic effect. It is also unknown whether there are differences in the effects of pulmonary rehabilitation under HFNC for each underlying disease.

Inclusion Criteria:

• Subjects with a minimum SPO2 of 88% or less during performing 6MWT under HFNC (FIO2 0.21 and a flow rate of 10L/min)
• Subjects who have been clinically stable for the last 2 weeks
• Subjects with written informed consent to participate in this study

Exclusion Criteria:

• Subjects with severe cardiovascular disease, liver disease, neurological disease, and renal failure
• Subjects who needed antimicrobial agent or steroid administration for pneumonia and exacerbation of respiratory disease in the last 2 weeks
• Subjects who cannot undergo 6MWT due to severe heart failure, arteriosclerosis obliterans or spinal disease