Since the outbreak of the coronavirus disease 2019 (COVID-19) pandemic, guidance (“Japanese Guide”) has been published by a working group of several academic societies and announced by the Ministry of Health, Labour, and Welfare. Steroids as a candidate treatment for COVID-19 were noted in the Japanese Guide. However, the prescription details for steroids, and whether the Japanese Guide changed its clinical practice, were unclear. This study aimed to examine the impact of the Japanese Guide on the trends in the prescription of steroids for COVID-19 inpatients in Japan. We selected our study population using Diagnostic Procedure Combination (DPC) data from hospitals participating in the Quality Indicator/Improvement Project (QIP). The inclusion criteria were patients discharged from hospital between January 2020 and December 2020, who had been diagnosed with COVID-19, and were aged 18 years or older. The epidemiological characteristics of cases and the proportion of steroid prescriptions were described on a weekly basis. The same analysis was performed for subgroups classified by disease severity. The study population comprised 8603 cases (410 severe cases, 2231 moderate II cases, and 5962 moderate I/mild cases). The maximum proportion of cases prescribed with dexamethasone increased remarkably from 2.5 to 35.2% in the study population before and after week 29 (July 2020), when dexamethasone was included in the guidance. These increases were 7.7% to 58.7% in severe cases, 5.0% to 57.2% in moderate II cases, and 1.1% to 19.2% in moderate I/mild cases. Although the proportion of cases prescribed prednisolone and methylprednisolone decreased in moderate II and moderate I/mild cases, it remained high in severe cases. We showed the trends of steroid prescriptions in COVID-19 inpatients. The results showed that guidance can influence drug treatment provided during an emerging infectious disease pandemic.