Provided is a method of forming a transition metal chalcogenide thin-film and the method includes preparing a first substrate having formed thereon a transition metal-containing precursor thin-film; displacing a second substrate separately with a constant distance from the first substrate by using a bridge unit while the second substrate is facing the first substrate, thereby securing a gas flowing path between the first substrate and the second substrate; heating the first and second substrates to a reaction temperature; and introducing a chalcogen-containing gas from an end of a reactor, such that the chalcogen-containing gas flows via the path.