A wavelength tunable laser is an indispensable light source for coherent optical communication systems. Multichannel frequency division multiplexing (FDM) systems in particular require the wide tuning range of more than 10 nm for a local oscillator; electrical tuning is a good prospect for this. Many types of electrical tunable laser have been proposed; however, their tuning ranges are not as wide as 5 nm. To increase the tuning range we have examined the composition of an InGaAsP guide layer from the point of view of the tuning range.