In this paper we discuss estimation of efforts needed to update programs according to a given requirement change. In the Object-Oriented prototyping development(shortly the OO prototyping), the requirement changes occur frequently and regularly. Thus a simple and fast estimation of efforts is strongly required by both developers and managers. However, existing estimation methods cannot be applied to the OO prototyping. Therefore we will try to propose a straightforward approach to effort estimation, which reflects the specific properties of the OO prototyping. First, we analyze the following characteristics of the OO prototyping: (1) updating activities consist of creation, deletion and modification, (2) the target to be updated has four kinds of types(void type, basic type, library type and custom type) and (3) the degree of information hiding is classified into private, protected and public. Then, we present a new formula $E(P,\sigma)$ to calculate the efforts needed to update a program $P$ according to a set of requirement changes $\sigma$. The formula $E(P,\sigma)$ includes weighting parameters: $w_{upd}$, $w_{type}$ and $w_{inf\mbox{-}h}$ according to the characteristics (1), (2) and (3), respectively. Finally, we conduct experimental evaluations by applying the formula $E(P,\sigma)$ to actual project data in a certain company. The evaluation results prove statistically to some extent the validity of the proposed approach.