Provided is a technique whereby drug-penetrability (permeability) can be increased in the nail suffering from onychomycosis and thus therapeutic effect can be promoted. A laser irradiation device for treating onychomycosis, comprising: a laser light source； a laser transmission means that is provided with a laser irradiation site； a controlling unit for controlling the laser output； and a fixation device for fixing the laser irradiation site in a sealed state to the nail to be irradiated with laser, wherein the laser irradiation site is fixed in a sealed state, with the aforesaid fixation device, to the site of the nail to be irradiated, and then the nail is irradiated with laser to thereby form a crack through which a therapeutic for onychomycosis is penetrated into the nail.
