The fertilizer conversion factor is used to calculate the amount of available nutrients in organic matter, and it is indispensable for estimating the total amount of chemical fertilizer to be applied to crop fields. Although a guideline to use the factor for single applications is available, it does not exist for consecutive applications to maize (Zea mays L.) fields. This study aims to determine the nitrogen fertilizer conversion factor for cattle manure and slurry applied in consecutive years to maize fields. We performed a burying field test in which nitrogen in manure and slurry was found to decompose over 5 years. In a 6-year cultivation test, the amount of hot-water extractable nitrogen, which is an indicator of soil nitrogen fertility, declined over the study period in some treatment plots. Moreover, the degree of this decrease was lower in some manure-applied plots. After the second year, the fertilizer conversion factors of manure and slurry from the start of consecutive applications were often higher than the current standard values (0.20 for manure and 0.40 for slurry), which were estimated after single application to maize fields. Additionally, the increase in fertilizer conversion factors of both manure and slurry may be related to the cumulative amount of organic nitrogen that was input. Our results indicate that the nitrogen fertilizer conversion factors in maize fields were 0.30 and 0.50 for manure and slurry, respectively after more than 3 or 4 consecutive years of application. Considering the organic nitrogen content in common manure and slurry from this area, we suggest that the use of a nitrogen fertilizer should be reduced after the 5th year of consecutive manure or slurry application.