An evaporative fuel processing device includes a canister, a purge path connected to the canister and an intake path that is connected to an internal combustion engine and through which purge gas flows from the canister to the intake path, and a flow control valve provided in the purge path to control the flow rate of the purge gas, wherein: the device further includes a first pressure detection unit that detects a first pressure at a first position between the canister and the flow control vale, and a concentration estimation unit that estimates a purge concentration of the purge gas flowing in the purge path; and the concentration estimation unit determines an increase in the first pressure generated by closing the flow control valve based on the detection value from the first pressure detection unit, and estimates the purge concentration based on the determined increase in the first pressure.
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