To provide a microreactor capable of preventing clogging of a fine channel or a liquid chromatograph even when two substrates where a recess for the fine channel or a recess for the liquid chromatograph is formed are stuck by an adhesive.

This microreactor comprises a lower substrate where protrusions are formed so as to enclose the recess for the fine channel having the width of 0.1-3,000 μm, and an upper substrate where a protrusion insertable into the recess for the fine channel is formed and recesses capable of inserting the protrusions formed on the lower substrate are formed on both sides across the protrusion. The microreactor is formed by inserting each protrusion into each recess on the other substrate and by laminating the lower substrate and the upper substrate, and has the fine channel having the width and the height of 0.1-3,000 μm, formed between the lower substrate and the upper substrate.