In a data processor including a master circuit that issues an access request and slave circuits that perform processing in response to the access request received from the master circuit, the disclosed invention enables the master circuit to access all data areas of the slave circuits even if the master circuit does not have an access command in data size suitable for accessing the slave circuit. An access size control unit that can convert access size input from the master circuit to data size in which the slave circuit accepts access is installed between the master circuit and the slave circuit. The access size control unit retains at least one address for access size setting. As the master circuit accesses the appropriate address for access size setting, the access size conversion procedure can be carried out.