An optical module capable of monitoring an inner temperature thereof by a simple arrangement is disclosed. The optical module installs an avalanche photodiode (APD). The APD generates the first photocurrent under a bias where the APD shows the multiplication factor thereof M equal to the unity, and the second photocurrent under another bias where the multiplication factor becomes greater than the unity. The operating temperature of the laser diode (LD) may be estimated from a ratio of the first photocurrent to the second photocurrent.