The present invention relates to a medicament for treating or preventing a disease involving Na channel, for example, neuropathic pain, nociceptive pain, inflammatory pain, small-fiber neuropathy, erythromelalgia, paroxysmal extreme pain disorder, dysuria, or multiple sclerosis, comprising a compound of formula (I) wherein R1a, R1b, R1c, and R1d are hydrogen, halogen, cyano, C1-4 alkyl, C1-4 alkoxy, etc., provided that at least one of R1a, R1b, R1c and R1d is the above C6-10 aryl, C6-10 aryloxy, etc., R2 and R3 are hydrogen, C1-6 alkyl, C3-10 cycloalkyl, etc., R4 is hydrogen, C1-6 alkyl, C3-7 cycloalkyl, etc., m is 0, 1, or 2, L is CR7R8, R7 and R8 are hydrogen, hydroxy group, C1-4 alkyl, C1-4 alkoxy, etc., or a pharmaceutically acceptable salt thereof.
