The present invention provides a glass preform heating furnace in which the occurrence of arching is suppressed. The glass preform heating furnace is equipped with a susceptor (3); a slit heater (4); an insulator; and a furnace body, wherein, in the case that the space between the slit heater (4) and the susceptor or between the slit heater (4) and the conductive member closest to the slit heater is D, that the maximum value of the electric field in this space is E1, that the number of the slits in the slit heater is N, that the slit width of the slit heater is S, and that the maximum value of the electric field in the slit space is E2, the values of D, N and S are set so that E1≧E2 is established.