  We report the measurement of the $\gamma p \rightarrow K^{+}\Lambda$ and
$\gamma p \rightarrow K^{+}\Sigma^{0}$ reactions at SPring-8. The differential
cross sections and photon-beam asymmetries are measured at forward $K^{+}$
production angles using linearly polarized tagged-photon beams in the range of
$E_{\gamma}=1.5$--3.0 GeV. With increasing photon energy, the cross sections
for both $\gamma p \rightarrow K^{+}\Lambda$ and $\gamma p \rightarrow
K^{+}\Sigma^{0}$ reactions decrease slowly. Distinct narrow structures in the
production cross section have not been found at $E_{\gamma}=1.5$--3.0 GeV. The
forward peaking in the angular distributions of cross sections, a
characteristic feature of $t$-channel exchange, is observed for the production
of $\Lambda$ in the whole observed energy range. A lack of similar feature for
$\Sigma^{0}$ production reflects a less dominant role of $t$-channel
contribution in this channel. The photon-beam asymmetries remain positive for
both reactions, suggesting the dominance of $K^{*}$ exchange in the $t$
channel. These asymmetries increase gradually with the photon energy, and have
a maximum value of +0.6 for both reactions. Comparison with theoretical
predictions based on the Regge trajectory in the $t$ channel and the
contributions of nucleon resonances indicates the major role of $t$-channel
contributions as well as non-negligible effects of nucleon resonances in
accounting for the reaction mechanism of hyperon photoproduction in this photon
energy regime.
