  The Berezinskii-Kosterlitz-Thouless-type continuous phase transition observed
in the three-spin interaction model is discussed. The relevant field theory
describes the topological defects involved and enables us to perform the
renormalization-group analysis. Based on it, we shall propose the
finite-size-scaling ansatz for the helicity modulus which exhibits the exponent
$\bar\nu=3/5$ for the correlation length in the disordered phase. We perform
the Monte Carlo simulations to confirm the ansatz. Also, we argue its relevance
to the ground-state phase transition in the quantum spin chain.
