PURPOSE: To obtain a new oxathiane derivative or its salt having a higher safety and a stronger wide-ranging antifungal activity and useful as an antifungal agent.

CONSTITUTION: An oxathiane derivative is represented by formula I {ph is phenyl. R1 and R2 are each H or a 1 to 6C alkyl; R3 and R4 are each H, an alkyl or CH2WR5 [W is NH, O, S(O)m or OC(O); R5 is H, an alkyl, a cycloalkyl or an aryl; (m) is 0 to 3. R5 is H in the case of m=3]; Y is N or CH; (n) is 0 to 2} or its salt, e.g. (2R*, 3R*)-2-(2,4-difluorophenyl)-3-methyl-2-(1H-1,2,4- triazol-1-ylmethyl)-1,4-oxathiane. In the case of n=0, the compound (formula Ia) of formula I can be synthesized by reacting a compound of formula II with phenylphosphine and azodicarboxylic acid diethyl. A therapeutic agent containing the compound of formula I as the active component has a special effect on local and deep mycosis.