1. An image decoder comprising: one or more processors configured to select a collocated picture, the collocated picture having a different temporal order from a current picture that includes a current prediction block, and the collocated picture comprising a temporal neighboring block which is used to derive a temporal merging candidate of the current prediction block, derive a spatial merging candidate by using a spatial neighboring block, derive the temporal merging candidate by using the temporal neighboring block, generate a merging candidate list including the spatial merging candidate and the temporal merging candidate, determine a motion vector and a reference picture index of the current prediction block by using the merging candidate list, the reference picture index of the current prediction block specifying a reference picture of the current prediction block, wherein the reference picture of the current prediction block has a different temporal order from the current picture and the collocated picture, obtain prediction samples of the current prediction block by using reconstruction samples within the reference picture specified by the reference picture index of the current prediction block, the reconstruction samples being indicated by the motion vector of the current prediction block, obtain residual samples of a current transform block by performing inverse-quantization and inverse-transform, and generate a reconstruct block by adding the prediction samples and the residual samples, wherein while a reference picture index of the spatial merging candidate is determined based on a reference picture of the spatial neighboring block, a reference picture index of the temporal merging candidate is set to a fixed value of zero, the reference picture index of the temporal merging candidate being used to determine the reference picture index of the current prediction block, but not used to determine the collocated picture of the current prediction block, and wherein the spatial neighboring block and the temporal neighboring block are determined based on: a position of a coding block, a size of a block on which a parallel merge processing is applicable, and a size of the coding block, wherein the coding block includes the current prediction block.