We present the ultra-deep Subaru narrowband imaging survey for Lyα emitters (LAEs) at z= 7.3 in the Subaru/XMM-Newton Deep Survey (SXDS) and Cosmic Evolution Survey (COSMOS) fields (∼ 0.5 deg 2) with a total integration time of 106 hr. Exploiting our new sharp bandwidth filter, NB101, installed on the Suprime-Cam, we have reached L (Lyα)= 2.4× 10 42 erg s− 1 (5σ) for z= 7.3 LAEs, about four times deeper than previous Subaru z≳ 7 studies, which allows us to reliably investigate the evolution of the Lyα luminosity function (LF) for the first time down to the luminosity limit same as those of Subaru z= 3.1–6.6 LAE samples. Surprisingly, we only find three and four LAEs in the SXDS and COSMOS fields, respectively, while one expects a total of∼ 65 LAEs by our survey in the case of no Lyα LF evolution from z= 6.6 to 7.3. We identify a decrease of the Lyα LF from z= 6.6 to 7.3 at the> 90% confidence level from our z= 7.3 Lyα LF with the best-fit Schechter parameters of $ L^{*} _ {\mathrm {Ly}\alpha}= 2.7^{+ 8.0} _ {-1.2}\times 10^{42}\\mathrm {erg}\\mathrm {s}^{-1} $