  Possibility of an axial vector isoscalar tetraquark $ud\bar{s}\bar{s}$ is
discussed. If a $f_1$ meson in the mass region $1.4-1.5$ GeV consists of four
quarks $ns\bar{n}\bar{s}$, the mass of the isoscalar
$ud\bar{s}\bar{s}$($\vartheta^+$-meson) state with $J^P=1^+$ is expected to be
lower than that of the $f_1$ meson. Within a flux-tube quark model, a possible
resonant state of $ud\bar{s}\bar{s}(J^{P}=1^{+})$ is suggested to appear at
$\sim$ 1.4 GeV with the width ${\cal{O}}(20\sim 50)$ MeV. We propose that the
$\vartheta^+$-meson is the good candidate for the tetraquark search, which
would be observed in the $K^+K^+\pi^-$ decay channel.
