The purpose of this study was to investigate whether self-efficacy for exercise decreased 12 weeks after educational intervention in diabetic patients and whether this had an effect on glycemic control. Thirty-eight diabetic patients underwent a 2-week educational program and were then followed for 12 weeks after discharge. The intervention program was delivered by several types of medical professionals. Lectures on exercise were given by a physical therapist, who tailored exercise instructions for individual patients. The assessment of self-efficacy for exercise was performed using a scale from a previous study, modified for Japanese people. The scale consisted of four sub-items: physical fatigue, mental stress, lack of time, and poor weather. Measurements were performed at the time of discharge and 12 weeks after discharge. Glycated hemoglobin (HbA1c) levels were assessed as an index of glycemic control, and diabetic status and social status factors were recorded. The physical fatigue and lack of time components of the self-efficacy for exercise scale were significantly decreased 12 weeks after discharge. Positive correlations were found between HbA1c levels at 12 weeks and duration of diabetes, number of educational admissions, and presence of diabetic retinopathy. The results suggest that the factors that are difficult to reproduce during hospital education (physical fatigue and lack of time) are the ones that decrease after discharge. To clarify the relationship between changes in self-efficacy for exercise and glycemic control, further studies with an extended follow-up period (24 weeks or 48 weeks) are needed.