Provided are novel polymer particles for carrying a physiologically active substance and a method of preparing the same. The polymer particles for carrying a physiologically active substance can provide an analytical reagent, which has high analytical precision and sensitivity and can be stably prepared; can easily and precisely control the amount of functional groups carrying the physiologically active substance; can introduce, onto the surface of latex particles, a hydrophilic compound for inhibiting a nonspecific reaction; and can be prepared to have a narrow and uniform particle size distribution. The polymer particles for carrying a physiologically active substance are obtained by polymerizing a monomer, a radical polymerization initiator, and an emulsifier, and the emulsifier is an amphiphilic block polymer of the general formula (1):