Mizuho Financial Group Inc. overtook Nomura Holdings Inc. as the No. 1 underwriter of corporate bonds in the first six months as yield premiums on corporate debt fell to their lowest level in eight years.

The nation's third-largest bank by market value helped underwrite 25 percent of this year's ¥4.16 trillion in note sales including offerings by Nissan Motor Co. and Mitsubishi Corp., according to data compiled by Bloomberg. Nomura, last year's leader, fell to third place behind Mitsubishi UFJ Financial Group Inc.

Japanese investors are chasing higher spreads by buying longer-term bonds and those of overseas issuers as unprecedented Bank of Japan stimulus and concerns the economy's rebound is stalling pushed down government debt yields. SoftBank Corp. was again the single biggest seller of notes during the period, providing underwriters with fees five times higher than the average paid by Toyota Motor Corp.