We developed a simple optical biosensing system using the count-based analysis method based on the selective light absorption and the specific wavelength filtering effect. The developed system used a gadget that consisted of a paper-based signal guide, a biosensing channel, and a smartphone. The paper-based signal guide was prepared by printing red patterns of various brightness against a dark background. Since a blue product was generated by the horseradish peroxidase (HRP)-based enzymatic reaction in the biosensing channel, the channel was used as a blue filter that can absorb the red light. When the red light reflected from the red pattern was absorbed by the channel, the pattern was observed as black. In other words, since the color of the patterns was assimilated with the background, the pattern seemed to disappear. Consequently, the amount of the blue product related to the concentration of target analyte can be measured by counting the number of observed patterns on the paper-based signal guide. In this study, the concentration of urinary C-telopeptide fragment of type II collagen (uCTX-II, 0-10 ng/mL) was measured using the developed system without complicated equipment. Therefore, we expect that the developed optical transducing system could be applied for point-of-care testing (POCT).