The fixed-bed reactor of the present invention which solves the above problems is a fixed-bed reactor comprising: a first catalyst layer filled with a first catalyst for producing acrolein from propylene; a second catalyst layer filled with a second catalyst for producing acrylic acid from acrolein; and an inert substance layer provided between the first catalyst layer and the second catalyst layer, and filled with an inert substance of a cylindrical shape having a surrounding wall in which an opening is formed. The process for producing acrylic acid of the present invention is a process for producing acrylic acid comprising the step of producing acrylic acid from propylene by using the fixed-bed reactor of the present invention. According to the present invention, since the inert substance of a cylindrical shape having a surrounding wall in which an opening is formed is used as an inert substance, increase of a pressure loss can be drastically suppressed, degradation of the catalyst due to by-produced high-boiling substances and the like can be suppressed, and further the yield of acrylic acid can be enhanced, as compared with the case of using conventional inert substances. Accordingly, it is possible to produce acrylic acid at a high yield stably for a long period.