PURPOSE: To simply and rapidly measure a particle distribution pitch of a charges in a moving layer in a blast furnace.

CONSTITUTION: A probe 3 is formed of light emitting fiber and light receiving fiber and has two optical cables 38, 29 in which a detecting field D is smaller than a diameter of a charge particle and which are separated at a distance ΔL of about the diameter of the particle. The probe 3 is inserted into a blast furnace with the direction of the ΔL as a perpendicular direction, and a charge dropping speed M is calculated from a phase difference between a light reception signal of the cable 38 of an upstream side and a light reception signal of the cable 39. On the other hand, a period Tphm of the signal of only the cable 38 is calculated, and a charge particle distribution pitch Pthm is calculated from the speed Vm and the period Tphm.