1. A microprogram controlling method for executing machine language instructions by using microprograms, said method comprising the steps of: decoding a machine language instruction in a decoding means, and obtaining information specifying a microprogram to be used for executing said machine language instruction, information about operands to be used for execution of said machine language instruction and information including an operational code which is common to machine language instructions differing only in an operand source; sending said information about said operands to operand controlling means and storing said information about said operands in said operand controlling means, said operand controlling means storing at least an immediate operand and an address of a memory as information about operands of a single machine language instruction at the same time; checking by said operand controlling means to see whether or not information about memory operands exists among said information about said operands stored in said operand controlling means; if it is judged by said checking step that information about a memory operand is held in said operand controlling means, carrying out a plurality of operand fetch operations for one microinstruction by accessing an external memory according to said information about said memory operands and fetching said memory operands for said operand controlling means; sending said information specifying said microprogram to microprogram storing means; reading a microinstruction constituting said microprogram from said microprogram storing means according to said information specifying said microprogram; sending an operand request signal which does not depend on the kind of operand from said microinstruction to said operand controlling means; and outputting a requested operand to a local bus from said operand controlling means according to said operand request signal.