An internal combustion engine (10) uses hydrogen as fuel. The internal combustion engine (10) includes a coupling passage (31, 32, 34, 35) that connects a crankcase (19) and a surge tank (60) to each other. A controller (100) executes a control of causing an air-fuel ratio of an air-fuel mixture to be lower when a target output of the internal combustion engine (10) is relatively high than when the target output is relatively low. The controller (100) executes a pressure reduction process of reducing a pressure in an intake passage (20, 29, 60) when the target output is less than a specific value. The pressure reduction process is a process of reducing the pressure in the intake passage (20, 29, 60) to be lower than that before the execution of the pressure reduction process.