PURPOSE: To obtain a pre-expander for a foamable resin adapted so that the expansion ratio may be corrected at once by changing the metering value of feed pellets according to the deviation when the signal sent from a metering device is compared with the set value, and the sent signal is outside the tolerance range of the set value.

CONSTITUTION: A feed metering hopper 8B is provided above a pre-expander body 2 for pre-expanding foamable resin pellets; a feed metering device 8F is provided on the feed metering hopper 8B; a bottom closing device 8C is provided on the bottom of the feed metering hopper 8B; a pre-expanded particle metering device 5 is provided adjacent to the pre-expander body 2 and meters the pre-expanded particles sent from the pre-expander body 2; the signal of the pre-expanded particles sent from the pre-expanded particle meter 5 is compared with the set expansion ratio in a controller 6; and when the sent value of the pre-expanded particles is outside the tolerance range of the set value, the feed rate of the feed is increased or decreased according to the deviation from the tolerance range by means of the feed metering device 8F.