In the present study, we investigated the acquisition of skill in the self-administration of insulin (by insulin pens) among 79 diabetes outpatients at Ehime University School of Medicine in order to evaluate the influence of such skill on glycemic control. The degree of skill acquisition by patients with poor glycemic control (HbA1C≥8.0) was significantly (p<0.05) lower than that by those with good control (HbA1C<8.0), and patients who had continuously used insulin pens over a 3-year period had higher rates of incorrect usage. In addition, the mean HbA1C of patients who kept the needle of the insulin pen pointing downwards for a certain period of time was significantly (p<0.05) lower than that for those who held the needle downwards for less than this period of time. However, HbA1C was not affected by skill in draining needles after pointing insulin pens downwards, knowledge of product name or injection units, shaking, or priming. These results indicated that the precise acquisition of skill in the self-administration of insulin is necessary to achieve good glycemic control and that keeping the needle pointing downwards for a sufficient period of time is the most important factor in the self-administration procedure. They also suggested that medical staff should keep a check on the skill of patients in the self-administration of insulin and repeatedly provide instruction on this to patients.