1. An arithmetic unit comprising: data input means for inputting data to be subjected to arithmetic processing; command input means for inputting a command for specifying processing on said data; arithmetic processing means for subjecting the data inputted by said data input means to arithmetic processing; data output means for outputting the data processed by the arithmetic processing means; output selecting means for selecting data to be outputted from the data output means; and control means for controlling said output selecting means such that only data allowed to be outputted is outputted from said data output means on the basis of the command inputted by said command input means.