This study assessed indicators of the need for insulin therapy and the effect of treatment on pregnancy outcomes in Japanese patients with gestational diabetes mellitus (GDM). All patients diagnosed with GDM were hospitalized for three days. Plasma glucose profiles in patients under strict dietary management and the characteristics of GDM patients with high daily glucose levels were investigated. Patients who failed to achieve glycemic targets were treated with insulin. Indicators of the need for insulin treatment were investigated. Pregnancy outcomes in patients prescribed dietary management and patients prescribed insulin treatment were compared. The study included 112 patients with GDM. GDM patients with high daily glucose levels in the hospital exhibited significantly higher 1-h and 2-h plasma glucose levels in oral glucose tolerance tests (OGTTs) at diagnosis. In our hospital, 102 GDM patients with singleton pregnancies were followed until delivery; 32 (31.3%) were treated with insulin. Univariate analysis identified significant associations of insulin requirement with family history of diabetes and with 1-h and 2-h OGTT values at diagnosis. Multivariate analysis showed that the 1-h OGTT plasma glucose level at diagnosis was an independent predictor of the need for insulin. In perinatal outcomes, insulin treatment was associated with low birth weight.