Provided is a technology for a gain-switching type transimpedance amplifier which prevents unnecessary gain switching that occurs when noise is received, and prevents reduction in sensitivity which occurs when a burst signal with a low input level is received after noise with a large input level is received. To provide this, the transimpedance amplifier (5) is provided with: a preamplifier (200) into which burst signals are inputted; an average detection start determination unit (300) which compares the output of the preamplifier (200) to a first threshold voltage; an average detection circuit (400) which detects and outputs the average of the output of the preamplifier (200) within a fixed period of time from the point of time when the output of the preamplifier (200) exceeds the first threshold voltage; and a gain-switching control unit (500) which determines whether to switch the gain of the preamplifier (200) based on the output of the average detection circuit (400).