The present disclosure relates to a condensate trap for a gas furnace for collecting and discharging condensate generated in a heat exchanger and an exhaust pipe, the condensate trap including: a first inlet through which the condensate generated in the heat exchanger is introduced; a second inlet through which the condensate generated in the exhaust pipe is introduced; a first passage through which the condensate introduced from the first inlet passes; a second passage through which the condensate introduced from the second inlet passes; a discharge port through which the condensate, having passed through the first passage and the second passage, is discharged outside; and a backflow prevention device disposed on the first passage and configured to prevent backflow of air, wherein the backflow prevention device includes: a housing; and a core which is movably disposed in the housing, and which in response to an amount of the condensate introduced from the first inlet being less than or equal to a predetermined amount, prevents backflow of the air by closing the first passage.