The protection of education funding has been one of the characteristics in Taiwan, but we don't have an academic reason for that. Therefore, we use cost function approach that is popular with foreign scholars and legislators to fulfill the incomplete part and take the elementary schools in Taipei for example to estimate the adequate funding. We find that school size and funding efficiency would affect adequate funding. The less school size, the more adequate funding they will need to attain the same student achievement compared with the same school size. However, if those schools use resource efficiently, the adequate funding can be reduced by 14% to 36%. We also find that the cause of not reaching adequate funding maybe lack funding or use resource inefficiently. We offer three recommendations and expect we can estimate the adequate funding for whole elementary schools in Taiwan after national education database is founded.