To provide a method of manufacturing a microfluid circuit which eliminates the need to position a mask, and is free of a position shift of the mask and low in manufacturing cost.

The present invention relates to the method of manufacturing the microfluid circuit in which a transparent upper plastic substrate is stacked on a light-absorptive lower plastic substrate, and the upper plastic substrate and lower plastic substrate are welded together by irradiating the lower plastic substrate with light through the upper plastic substrate, and the method is characterized in having a microchannel on a bottom surface of the upper plastic substrate and/or a top surface of the lower plastic substrate, and also in including a process of forming, on the upper plastic substrate, a light attenuation region in which an amount of light transmitted to the microchannel decreases when the lower plastic substrate is irradiated with the light through the upper plastic substrate.