It is intended to provide a host microorganism whereby the expression of a gene encoding a target protein can be effectively elevated in the case of transferring the gene. It is also intended to provide a transformant whereby a target protein can be produced at a high efficiency. It is further intended to provide a production process whereby a target protein can be produced at a high productivity. A microorganism which belongs to Eumycota and lacks the major isomaltose synthase. A gene encoding a target protein is transferred into the microorganism to give a transformant. Then the transformant is cultured under conditions allowing the expression of the transferred gene to thereby produce the protein.