  The Kondo effect in coupled quantum dots is investigated from the viewpoint
of transmission spectroscopy using the slave-boson formalism of the Anderson
model. The antiferromagnetic spin-spin coupling $J$ between the dots is taken
into account. Conductance $G$ through the dots connected in a series is
characterized by the competition between the dot-dot tunneling coupling $V_{C}$
and the level broadening $\Delta$ in the dots (dot-lead coupling). When
$V_{C}/\Delta < 1$, the Kondo resonance is formed between each dot and lead,
which is replaced by a spin-singlet state in the dots at low gate voltages. The
gate voltage dependence of $G$ has a sharp peak of $2 e^2/h$ in height in the
crossover region between the Kondo and spin-singlet states. The sharp peak of
$G$ survives when the energy levels are different between the dots. When $V_{C}
/ \Delta > 1$, the "molecular levels" between the Kondo resonant states appear;
the Kondo resonant peaks are located below and above the Fermi level in the
leads at low gate voltages. The gate voltage dependence of $G$ has a broad
peak, which is robust against $J$. The broad peak splits into two peaks when
the energy levels are different, reflecting the formation of the asymmetric
molecular levels between the Kondo resonant states.
