PURPOSE: To check characteristics of a heterogeneous optical film by forming a film for monitoring on the surface of a base by a deposited material settling on the surface of the base through a slit port.

CONSTITUTION: A port 38 for monitoring is arranged by the side of a main port 19 and a zone 44 for monitoring is formed by the side of a main zone 43. When the film for monitoring is formed, a movable shield plate 40 for monitoring is put closer to a fixed shield plate 39 for monitoring to narrow down the width of the slit port 38 sufficiently, and imaginary layers, therefore, decrease in thickness, so variation in thickness-directional characteristics of a heterogeneous optical film in the main zone 43 coincides substantially with variation in breadthwise characteristics of the film for monitoring formed in the zone 44 for monitoring. Then, the variation in the breadthwise characteristics of the film for monitoring is monitored to obtain information on variation in thickness-directional characteristics of the heterogeneous optical film.