We present a microfluidic device for on-chip analysis of low-concentration protein biomarkers. A new detection method, based on the magnetic capture of proteins via superparamagnetic beads and counting the surface coverage of the latter is presented. The preconcentration of the proteins on the detection area is realized in a reasonable time using a dedicated on-chip chaotic mixing chamber. With the presented device, 1 ng/mL (~ 6.5 pM) biotinylated goat anti-streptavidin molecules have been easily detected on the magnetic beads covered with streptavidin molecules. Low back-ground signal levels allow sensitive protein detection, even in a few microliters of sample.