Recent analyses of long duration event (LDE) flares indicate successive occurrences of magnetic reconnection and resultant energy release in the decay phase. However, quantitative studies of the energy release rate and the reconnection rate have not yet been made. In this paper we focus on the decay phase of an LDE flare on 1997 May 12 and derive the energy release rate H and the reconnection rate MA=vin/vA, where vin is the inflow velocity and vA is the Alfvén velocity. For this purpose, we utilize a method to determine vin and the coronal magnetic field Bcorona indirectly, using the following relations:H=2B2corona/4vinAr,Bcoronavin=Bfootvfoot,where Ar, Bfoot, and vfoot are the area of the reconnection region, the magnetic field strength at the footpoints, and the separation velocity of the footpoints, respectively. Since H, Ar, vfoot, and Bfoot are obtained from the Yohkoh Soft X-Ray Telescope data and a photospheric magnetogram, vin and Bcorona can be determined from these equations. The results are as follows: H is ~1027 ergs s-1 in the decay phase. This is greater than 1/10th of the value found in the rise phase. MA is 0.001-0.01, which is about 1 order of magnitude smaller than found in previous studies. However, it can be made consistent with the previous studies under the reasonable assumption of a nonunity filling factor. Bcorona is found to be in the range of 5-9 G, which is consistent with both the potential extrapolation and microwave polarization observed with the Nobeyama Radioheliograph.