The present invention provides a natural yeast extract having a high-impact first taste by containing glutamic acid in a large amount. Further, the present invention provides a yeast extract having strong umami by containing 5′-guanylic acid or 5′-inosinic acid in a large amount. In addition, the present invention provides a yeast mutant accumulating glutamic acid, glutamine, and ribonucleic acid in a large amount for obtaining the yeast extract.A yeast mutant inducing a natural mutation to which organic acid-tolerance or organic acid analogue-tolerance is imparted accumulates free glutamic acid and glutamine in such a significant amount as a total amount of 10% by weight or more in a cell, and further accumulates ribonucleic acid in an amount of 5% by weight or more. A yeast extract produced using the above yeast mutant contains L-glutamic acid in an amount of 20% by weight or more, and further contains 5′-IG in an amount of 3% by weight or more.