Clinical trials are a crucial step in the development of new medical technologies. Obtaining informed consent (IC) is a key component of clinical trials, which ensures that potential participants have the necessary information for decision-making. While patients' involvement is essential to improve the systems and circumstances of clinical trials, their perspectives and experiences are rarely shared with others. This study aims to investigate patients' perspectives and experiences of IC in clinical trials in Japan. A mixed method approach was adopted, including an internet-based survey of 21,502 patients and in-depth interviews of 41 patients. In the survey, 12,506 responses were analyzed, focusing on patients' understanding of IC. Among the 12,506 patients, 2,320 had some experience of clinical trials. Among the 888 patients who remembered receiving an explanation from the medical staff, 93.6% responded that it was easy to understand. Most respondents (84.5%) who had received an IC form read it again and 54.5% did not spend much time thinking before deciding whether to participate. From the in-depth interviews, we identified the following: the timing of decision-making and the meaning of IC for the patients. The patients made their “informal” decision prior to the “formal” IC process. They confirmed their “informal” decision during the IC process by collecting important information regarding the health risk of participation. From the patients' perspective, the timing of decision-making is outside the IC process. While patients make an informal decision, both medical staff and patients should recognize that written consent is the formal decision.