The earliest (z>8) supermassive black holes hold crucial information regarding the formation mechanism of the first generation black hole seeds. We propose to determine the redshifts of five radio quasar candidates at zphot=7.4-9.2, selected through the combination of deep Subaru/HSC optical, near-IR, and 3GHz VLASS radio surveys. These candidates exhibit significant 1.5GHz+3GHz radio emissions but lack emissions in the HSC optical (HSC-dark). Their emission starts to become evident in the J-band (referred to as y-dropouts). The IR color of these candidates suggests a dust-reddened feature with an estimated Av of 1-5 mag, classifying them as prominent candidates of dust-reddened radio quasar at z=7.4–9.2. We propose to perform Subaru/MOIRCS J-band spectroscopy, providing access to Lyman break, associated Lyα emission, and bright CIV emission. Upon the confirmation of their redshifts, the candidates will become the most distant radio quasars at z=7.4–9.2. Furthermore, assuming the Eddington-limit, it will make the first confirmation of extreme BH mass accretion event associated with radio emission in this early Universe at z=7.4–9.2.
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