A base station apparatus comprising: a configuration section that is adapted to configure a first search space and a second search space in physical resource blocks, RBs, distributed along frequency axis, within a data region, each of the first search space and the second search space being formed by a plurality of control channel elements and being a candidate to which control information for at least one terminal apparatus is allowed to be assigned; and a transmission section that is adapted to transmit the control information to be assigned in the first search space and/or the second search space, wherein the control channel elements of the first search space or the second search space are numbered in the first search space or the second search space, respectively, with indices; the control information for a terminal apparatus in one of the search spaces is to occupy one or several consecutive control channel elements, wherein an index of the first of said one of several consecutive control channel elements is associated on a one-to-one basis with an uplink resource to be used by the respective terminal to transmit a response signal to downlink data, characterized in that the control channel elements forming the first search space are numbered by the indices in an ascending order of the RBs, the control channel elements forming the second search space and corresponding to the RBs not included in the first search space, are numbered by the indices that are larger than the indices of the control channel elements forming the first search space, and the control channel elements forming the second search space and corresponding to the RBs included in the first search space are numbered by the indices that are same as the indices by which the control channel elements forming the first search space and corresponding to the same RBs are numbered. a configuration section that is adapted to configure a first search space and a second search space in physical resource blocks, RBs, distributed along frequency axis, within a data region, each of the first search space and the second search space being formed by a plurality of control channel elements and being a candidate to which control information for at least one terminal apparatus is allowed to be assigned; and a transmission section that is adapted to transmit the control information to be assigned in the first search space and/or the second search space, wherein the control channel elements of the first search space or the second search space are numbered in the first search space or the second search space, respectively, with indices; the control information for a terminal apparatus in one of the search spaces is to occupy one or several consecutive control channel elements, wherein an index of the first of said one of several consecutive control channel elements is associated on a one-to-one basis with an uplink resource to be used by the respective terminal to transmit a response signal to downlink data, characterized in that the control channel elements forming the first search space are numbered by the indices in an ascending order of the RBs, the control channel elements forming the second search space and corresponding to the RBs not included in the first search space, are numbered by the indices that are larger than the indices of the control channel elements forming the first search space, and the control channel elements forming the second search space and corresponding to the RBs included in the first search space are numbered by the indices that are same as the indices by which the control channel elements forming the first search space and corresponding to the same RBs are numbered.