  A microwave kinetic inductance detector (MKID) is a cutting-edge
superconducting detector. It comprises a resonator circuit constructed with a
superconducting film on a dielectric substrate. To expand its field of
application, it is important to establish a method to suppress the two-level
system (TLS) noise that is caused by the electric fluctuations between the two
energy states at the surface of the substrate. The electric field density can
be decreased by expanding the strip width (S) and gap width from the ground
plane (W) in the MKID circuit, allowing the suppression of TLS noise. However,
this effect has not yet been confirmed for MKIDs made with niobium films on
silicon substrates. In this study, we demonstrate its effectiveness for such
MKIDs. We expanded the dimension of the circuit from (S, W) = (3.00 $\mu$m,
4.00 $\mu$m) to (S, W) = (5.00 $\mu$m, 23.7 $\mu$m), and achieved an increased
suppression of 5.5 dB in TLS noise.
