In the step (S102) of drying, grinding and dissociating the seaweed fiber into fine fiber, the primarilycrushed seaweed fiber is grindingdissociated into fine fiber particles using a hightemperature grinder. At this time, the seaweed fiber particles are grinded with the hightemperature grinder at 6,000 rpm for 30 seconds or more and are passed through an 80 micrometer sieve.