The invention provides a heat treatment device capable of preventing generation of tar in an exhaust pipe. This device comprises a collective exhaust pipe (200), a first furnace (100a) and a second furnace (100b). The first furnace (100a) is connected to the upstream side of gas flow in the collective exhaust pipe (200), and the second furnace (100b) is connected to the downstream side of gas flowin the collective exhaust pipe (200). The sectional area of the collective exhaust pipe (200) satisfies the equation of S<2>/S<1>=(H<2>/H<1>) (0.1<=a<=0.9) when the sectional area in the connecting part of the exhaust pipe (200) with the first furnace (100a) is S<1>, the sectional area in the connecting part of the exhaust pipe (200) with the second furnace (100b) is S<2>, and the internal height of the first furnace (100a) is H<1>, and the total of the internal height of the first furnace (100a) and the internal height of the second furnace (100b) is H<2>.