Several clinical trials have demonstrated that linkage to an outreach service can prevent prolonged length of stay of patients at psychiatric hospitals. However, there has been no investigation of the association between length of stay in psychiatric hospital and regional supply of outreach services using national case mix data. The aim of this study was to clarify the relationship between length of stay in psychiatric hospital and regional supply of outreach services. We used data from the National Patient Survey in Japan, a nationally representative cross-sectional survey of inpatient care conducted every three years from 1996 to 2014. Data from 42,268 patients with schizophrenia who had been admitted to psychiatric hospitals were analyzed. After controlling for patient and regional characteristics, patients in regions with fewer number of visits for psychiatric nursing care at home had significantly longer length of stay in psychiatric hospitals. This finding implies that enhancement of the regional supply of outreach services would prevent prolonged length of stay in psychiatric hospitals.