A process for producing a hydrocarbon oil by a Fischer-Tropsch synthesis reaction using a slurry bubble column reactor that retains a slurry containing catalyst particles and liquid hydrocarbons inside thereof and that has a gaseous phase portion at an upper portion of the slurry, the process comprising: a step of flowing the slurry through a filter provided inside and/or outside of the reactor to separate the slurry into catalyst particles and heavy liquid hydrocarbons and to take out the heavy liquid hydrocarbons; a backwashing step of flowing liquid hydrocarbons through the filter in a direction opposite to the flowing direction of the slurry to return the catalyst particles accumulated on the filter into a slurry bed in the reactor, the process being characterised in that it comprises a monitoring step of estimating a content of particulates having a particle size which is equal to or smaller than the predetermined particle size in the slurry by a method comprising, based on a correlation between a visible light transmittance and a content of solid particles having the predetermined particle size or less at a temperature at which hydrocarbons including a wax are liquefied when the solid particles having the predetermined particle size or less are dispersed in the hydrocarbons, estimating a content of particulates having a particle size which is equal to or smaller than the predetermined particle size in the slurry from a visible light transmittance of a supernatant part when the slurry is left to stand at the temperature, wherein a frequency of performing the backwashing step or a time for replacing or washing the filter is determined based on the estimation result of a content of particulates having a particle size which is equal to or smaller than the predetermined particle size in the slurry obtained in the monitoring step. a step of flowing the slurry through a filter provided inside and/or outside of the reactor to separate the slurry into catalyst particles and heavy liquid hydrocarbons and to take out the heavy liquid hydrocarbons; a backwashing step of flowing liquid hydrocarbons through the filter in a direction opposite to the flowing direction of the slurry to return the catalyst particles accumulated on the filter into a slurry bed in the reactor, the process being characterised in that it comprises a monitoring step of estimating a content of particulates having a particle size which is equal to or smaller than the predetermined particle size in the slurry by a method comprising, based on a correlation between a visible light transmittance and a content of solid particles having the predetermined particle size or less at a temperature at which hydrocarbons including a wax are liquefied when the solid particles having the predetermined particle size or less are dispersed in the hydrocarbons, estimating a content of particulates having a particle size which is equal to or smaller than the predetermined particle size in the slurry from a visible light transmittance of a supernatant part when the slurry is left to stand at the temperature, wherein a frequency of performing the backwashing step or a time for replacing or washing the filter is determined based on the estimation result of a content of particulates having a particle size which is equal to or smaller than the predetermined particle size in the slurry obtained in the monitoring step.