Hi Ridwan,
According to the Service Manual, the RQ-2102 is a monaural recorder, meaning it only records a single channel. So yes, you'll only get sound on one channel of your stereo headphones unless you use an adapter that connects that single output channel to both of the stereo channels. It's called a mono to stereo adapter; you'll need the 3.5 mm size like this one.
[link|https://www.monoprice.com/product?p_id=7128|Monoprice 3.5mm TS Mono Plug to 3.5mm TRS Stereo Jack Adapter, Gold Plated - Monoprice.com|new_window=true]
With one of those, you'll hear sounds in both ears instead of just one.