Recently, application of the contingent valuation method (CVM) to health care is increasing to measure the willingness to pay (WTP) for specific medical services. In this study, we measured WTP for the outpatient treatment of hypertension and inpatient treatment of myocardial infarction (MI) in Japan's healthcare system, using CVM via an Internet questionnaire survey in 547 citizens aged 40 to 49 years. WTP was measured with the payment cards method from an ex post consumer based perspective. The payment vehicle was out-of-pocket copayment under public medical insurance. The participants were asked their preferences with respect to medical institutions, and 3 comprehensive characteristics were extracted from the requested information by principal component analysis. Categorical regression was performed to analyze the factors affecting WTP. The mean WTP for hypertension treatment was $75.03/month, and that for the treatment of MI was $8,928.70 ($1 = 105 Japanese yen). WTP for hypertension treatment was significantly high in married males and the group without symptoms, but was not associated with income. WTP for the treatment of MI was significantly high in the high-income group, married males, and the group with symptoms. Among the 3 principle components, "objective evaluation" was significantly associated with WTP for the treatment of MI. As for serious diseases such as MI, the income-associated differences in WTP suggest the necessity for reinforcement of the safety net for the low-income group. Although asymptomatic, hypertension requires continuous treatment. For such diseases, uniformly low copayment should be established irrespective of annual income.