We evaluated the peptide-specific immunoglobulin G (IgG) response and the safety of the personalized peptide vaccine in four children with refractory solid cancer. Although the pre-vaccination IgG responses were suppressed compared to those observed in children with upper respiratory tract infection, IgG levels against the vaccinated peptides after 12 times vaccination increased in all the three cases tested. Vaccination-related adverse effects were grade 1 injection-site local skin lesions. One patient had maintained remission for 37 months. The remaining three patients exhibited progressive disease. These results indicate the next steps in phase II studies considering their immune boosting effect and safety.