A fabrication method of a microfluidic circuit is provided, dispensable of mask registration, absent of deviation in mask positioning, and inexpensive in fabrication cost. The fabrication method of a microfluidic circuit has a light transmissive substrate stacked on a light absorptive substrate, and the light transmissive substrate welded with the light absorptive substrate for bonding by directing light through the light transmissive substrate. The microfluidic circuit includes a microchannel at the bottom face of the light transmissive substrate and/or at the top face of the light absorptive substrate. The method includes the step of forming, at the light transmissive substrate, a light attenuation region attenuating transmittance of light towards the microchannel when light is directed through the light transmissive substrate.