To make alterable the number of test trays circularly conveyed into a constant temperature oven by providing a buffer stage for temporarily containing an electronic component conveying medium.

A buffer stage CRB is provided between positions CR6 and CR6 of an IC carrier CR in a test chamber. The stage CRB is a stage for optimizing the number of IC carriers CR to be conveyed through a circulating route for starting from the CR1 and returning to CR1 and temporarily taking out the IC carrier CR from the route and storing it. If temperature conditions are severe and a soak time is longer than a test time of a test head 302, the stage CRB does not store the carrier C but circulates the carrier CR of full number, and shortens the time of waiting the head 302. If the test time is loner thin the soak time, several number of the carriers CR are stored in the stage CRB, the retaining carriers CR are circulated to prevent a loss of thermal energy in the test chamber.