To reduce scale of circuits of a data storage circuit in which the data width of a data input and a data output is freely changeable up to several times.

In this data processor, memory cells from and to which bit data having (a) bits are read and written are selected in a cell selecting circuit according to address data, but, a number of pieces changeover circuit changes over and sets the number of pieces of memory cells according to a selection signal. As a result, it is possible to change over and set the data width of the data input and the data output having (a) bits to B times with a piece of a data storage circuit and thus it is not necessary to provide B pieces of data storage circuit having data widths of (a) bits in parallel in the data processor.