A chemical synthesis device includes a reaction vessel, a waste liquid tank and an intermediate container. A chemical is supplied to the reaction vessel. The waste liquid tank is configured to hold waste liquid discharged from the reaction vessel. The intermediate container has a smaller capacity than the waste liquid tank. The intermediate container is provided between the reaction vessel and the waste liquid tank, via a waste liquid pipe. The pressure of the intermediate container is adjusted to adjust the flow supplied to the reaction vessel and the flow discharged from the reaction vessel.