To facilitate phase adjustment in an external resonator type wavelength-variable laser. An external resonator type wavelength-variable laser (50) includes a semiconductor light amplifier (1), a wavelength selection filter (3) having periodic frequency characteristics, an external resonator (6), and a wavelength-variable filter (4). Δf is divided into one or more regions. The following conditions are satisfied for one Δfi region: mj=(j×Δffs)/ΔfFP where Δffs: a period of the wavelength selection filter, ΔfFP: a Fabry-Perot mode interval dependent on a length of the external resonator, j: an integer not smaller than 1 and not larger than (Δfi/Δffs). A coefficient Mj is an integer obtained by rounding off the first digit after decimal point of the coefficient mj. A coefficient Ni,j is an integer obtained by discarding the first digit after decimal point of Δfi/(j×Δffs). In this regard, a relation of (|Ni,j×2π(Mj−mj)|